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>

web/app/public/index.html

@@ -3,7 +3,7 @@
   <head>
     <meta charset="utf-8" />
     <script type="text/javascript">
-      window.config = {logo: "{{ .UI.Logo }}", header: "{{ .UI.Header }}", dashboardHeading: "{{ .UI.DashboardHeading }}", dashboardSubheading: "{{ .UI.DashboardSubheading }}", link: "{{ .UI.Link }}", buttons: [], maximumNumberOfResults: "{{ .UI.MaximumNumberOfResults }}", defaultSortBy: "{{ .UI.DefaultSortBy }}", defaultFilterBy: "{{ .UI.DefaultFilterBy }}"};{{- range .UI.Buttons}}window.config.buttons.push({name:"{{ .Name }}",link:"{{ .Link }}"});{{end}}
+      window.config = {logo: "{{ .UI.Logo }}", header: "{{ .UI.Header }}", dashboardHeading: "{{ .UI.DashboardHeading }}", dashboardSubheading: "{{ .UI.DashboardSubheading }}", link: "{{ .UI.Link }}", buttons: [], maximumNumberOfResults: "{{ .UI.MaximumNumberOfResults }}", defaultSortBy: "{{ .UI.DefaultSortBy }}", defaultFilterBy: "{{ .UI.DefaultFilterBy }}", loginSubtitle: "{{ .UI.LoginSubtitle }}"};{{- range .UI.Buttons}}window.config.buttons.push({name:"{{ .Name }}",link:"{{ .Link }}"});{{end}}
       // Initialize theme immediately to prevent flash
       (function() {
         const themeFromCookie = document.cookie.match(/theme=(dark|light);?/)?.[1];