[feature] Add global instance CSS customization setting (#3352)

Allow instance admins to add custom CSS that will affect
every page of their instance.

This is done with a new CustomCSS instance setting that
works pretty much exactly like the Users CustomCSS property.
This custom CSS is then requested for every page load.
User styles/themes take precedence over this CSS.

Co-authored-by: tobi <tobi.smethurst@protonmail.com>

internal/web/thread.go

@@ -115,7 +115,7 @@ func (m *Module) threadGETHandler(c *gin.Context) {
 	}
 
 	// Prepare stylesheets for thread.
-	stylesheets := make([]string, 0, 5)
+	stylesheets := make([]string, 0, 6)
 
 	// Basic thread stylesheets.
 	stylesheets = append(
@@ -131,6 +131,7 @@ func (m *Module) threadGETHandler(c *gin.Context) {
 	if theme := targetAccount.Theme; theme != "" {
 		stylesheets = append(
 			stylesheets,
+			instanceCustomCSSPath,
 			themesPathPrefix+"/"+theme,
 		)
 	}