[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/config/config.go

@@ -73,6 +73,7 @@ type Configuration struct {
 	AccountsRegistrationOpen bool `name:"accounts-registration-open" usage:"Allow anyone to submit an account signup request. If false, server will be invite-only."`
 	AccountsApprovalRequired bool `name:"accounts-approval-required" usage:"Do account signups require approval by an admin or moderator before user can log in? If false, new registrations will be automatically approved."`
 	AccountsReasonRequired   bool `name:"accounts-reason-required" usage:"Do new account signups require a reason to be submitted on registration?"`
+	AccountsAllowCustomCSS   bool `name:"accounts-allow-custom-css" usage:"Allow accounts to enable custom CSS for their profile pages and statuses."`
 
 	MediaImageMaxSize        int `name:"media-image-max-size" usage:"Max size of accepted images in bytes"`
 	MediaVideoMaxSize        int `name:"media-video-max-size" usage:"Max size of accepted videos in bytes"`