[feature] Allow users to set custom css for their profiles + threads (#808)

* add custom css account property + db func to fetch

* allow account to get/set custom css

* serve custom css for an account

* go fmt

* use monospace for customcss, add link

* add custom css to account cache

* fix broken field

* add custom css docs to user guide

* add `accounts-allow-custom-css` config flag

* add allow custom css to /api/v1/instance response

* only show/set custom css if allowed to do so

* only set/serve custom account css if enabled

* update swagger docs

* chain promise

* make bool a bit clearer

* use cache for GetAccountCustomCSSByUsername

internal/api/mime.go

@@ -31,4 +31,5 @@ const (
 	MultipartForm     MIME = `multipart/form-data`
 	TextXML           MIME = `text/xml`
 	TextHTML          MIME = `text/html`
+	TextCSS           MIME = `text/css`
 )