feat(ui): Add customizable OIDC login screen branding (#1583)

* feat(ui): Add customizable OIDC login screen branding

As per https://github.com/TwiN/gatus/discussions/1579, this PR
allows for some customizations in the OIDC login screen:

- If a logo is set it will be displayed alongside the Gatus one
- New ui config `ui.login-subtitle`: customises the message in
  the screen
- If set, use `ui.header` will be used to customise the title

I haven't commited the web static assets. I believe that's
triggered with a comment? let me know if I should be doing it.

Also please clarify if I've forgotten about something here,
this is my first pr.

* chore(ui): Regenerate static assets

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: TwiN <twin@linux.com>

config/ui/ui.go

@@ -23,6 +23,7 @@ const (
 	defaultCustomCSS            = ""
 	defaultSortBy               = "name"
 	defaultFilterBy             = "none"
+	defaultLoginSubtitle        = "System Monitoring Dashboard"
 )
 
 var (
@@ -48,6 +49,7 @@ type Config struct {
 	DarkMode                *bool    `yaml:"dark-mode,omitempty"`              // DarkMode is a flag to enable dark mode by default
 	DefaultSortBy           string   `yaml:"default-sort-by,omitempty"`        // DefaultSortBy is the default sort option ('name', 'group', 'health')
 	DefaultFilterBy         string   `yaml:"default-filter-by,omitempty"`      // DefaultFilterBy is the default filter option ('none', 'failing', 'unstable')
+	LoginSubtitle           string   `yaml:"login-subtitle,omitempty"`         // LoginSubtitle is the subtitle displayed on the OIDC login page
 	//////////////////////////////////////////////
 	// Non-configurable - used for UI rendering //
 	//////////////////////////////////////////////
@@ -95,6 +97,7 @@ func GetDefaultConfig() *Config {
 		DarkMode:               &defaultDarkMode,
 		DefaultSortBy:          defaultSortBy,
 		DefaultFilterBy:        defaultFilterBy,
+		LoginSubtitle:          defaultLoginSubtitle,
 		MaximumNumberOfResults: storage.DefaultMaximumNumberOfResults,
 		Favicon: Favicon{
 			Default:   defaultFavicon,
@@ -143,6 +146,9 @@ func (cfg *Config) ValidateAndSetDefaults() error {
 	} else if cfg.DefaultFilterBy != "none" && cfg.DefaultFilterBy != "failing" && cfg.DefaultFilterBy != "unstable" {
 		return ErrInvalidDefaultFilterBy
 	}
+	if len(cfg.LoginSubtitle) == 0 {
+		cfg.LoginSubtitle = defaultLoginSubtitle
+	}
 	if len(cfg.Favicon.Default) == 0 {
 		cfg.Favicon.Default = defaultFavicon
 	}

config/ui/ui_test.go

@@ -50,6 +50,9 @@ func TestConfig_ValidateAndSetDefaults(t *testing.T) {
 		if cfg.Favicon.Size32x32 != defaultFavicon32 {
 			t.Errorf("expected favicon to be %s, got %s", defaultFavicon32, cfg.Favicon.Size32x32)
 		}
+		if cfg.LoginSubtitle != defaultLoginSubtitle {
+			t.Errorf("expected LoginSubtitle to be %s, got %s", defaultLoginSubtitle, cfg.LoginSubtitle)
+		}
 	})
 	t.Run("custom-values", func(t *testing.T) {
 		cfg := &Config{
@@ -62,6 +65,7 @@ func TestConfig_ValidateAndSetDefaults(t *testing.T) {
 			Link:                "https://example.com",
 			DefaultSortBy:       "health",
 			DefaultFilterBy:     "failing",
+			LoginSubtitle:       "Welcome",
 		}
 		if err := cfg.ValidateAndSetDefaults(); err != nil {
 			t.Error("expected no error, got", err.Error())
@@ -93,6 +97,9 @@ func TestConfig_ValidateAndSetDefaults(t *testing.T) {
 		if cfg.DefaultFilterBy != "failing" {
 			t.Errorf("expected defaultFilterBy to be preserved, got %s", cfg.DefaultFilterBy)
 		}
+		if cfg.LoginSubtitle != "Welcome" {
+			t.Errorf("expected LoginSubtitle to be preserved, got %s", cfg.LoginSubtitle)
+		}
 	})
 	t.Run("partial-custom-values", func(t *testing.T) {
 		cfg := &Config{
@@ -119,6 +126,9 @@ func TestConfig_ValidateAndSetDefaults(t *testing.T) {
 		if cfg.Description != defaultDescription {
 			t.Errorf("expected description to use default, got %s", cfg.Description)
 		}
+		if cfg.LoginSubtitle != defaultLoginSubtitle {
+			t.Errorf("expected LoginSubtitle to use default, got %s", cfg.LoginSubtitle)
+		}
 	})
 }
 
@@ -181,6 +191,9 @@ func TestGetDefaultConfig(t *testing.T) {
 	if defaultConfig.DefaultFilterBy != defaultFilterBy {
 		t.Error("expected GetDefaultConfig() to return defaultFilterBy, got", defaultConfig.DefaultFilterBy)
 	}
+	if defaultConfig.LoginSubtitle != defaultLoginSubtitle {
+		t.Error("expected GetDefaultConfig() to return defaultLoginSubtitle, got", defaultConfig.LoginSubtitle)
+	}
 }
 
 func TestConfig_ValidateAndSetDefaults_DefaultSortBy(t *testing.T) {