[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/domain-blocklist.go

@@ -67,7 +67,7 @@ func (m *Module) domainBlockListGETHandler(c *gin.Context) {
 		Template:    "domain-blocklist.tmpl",
 		Instance:    instance,
 		OGMeta:      apiutil.OGBase(instance),
-		Stylesheets: []string{cssFA},
+		Stylesheets: []string{cssFA, instanceCustomCSSPath},
 		Javascript:  []string{jsFrontend},
 		Extra:       map[string]any{"blocklist": domainBlocks},
 	}