[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/cache/account.go

@@ -131,6 +131,7 @@ func copyAccount(account *gtsmodel.Account) *gtsmodel.Account {
 		Sensitive:               copyBoolPtr(account.Sensitive),
 		Language:                account.Language,
 		StatusFormat:            account.StatusFormat,
+		CustomCSS:               account.CustomCSS,
 		URI:                     account.URI,
 		URL:                     account.URL,
 		LastWebfingeredAt:       account.LastWebfingeredAt,