Allow for user specific themes (#5668)

* add migration and basic UI for changing a user's theme

* update user themem

* use right text on button

* load theme based on users' selection

* load theme based on users' selection in pwa too

* update sample config

* delete older theme loading

* implement AfterLoad to set users' theme properly

* set up default theme when creating a user. This uses the installation wide theme

* use flash messages for error

* set default theme when creating a user from the cli

* fix @lunny review

modules/setting/defaults.go

@@ -8,4 +8,5 @@ var (
 	defaultLangs                             = strings.Split("en-US,zh-CN,zh-HK,zh-TW,de-DE,fr-FR,nl-NL,lv-LV,ru-RU,uk-UA,ja-JP,es-ES,pt-BR,pl-PL,bg-BG,it-IT,fi-FI,tr-TR,cs-CZ,sr-SP,sv-SE,ko-KR", ",")
 	defaultLangNames                         = strings.Split("English,简体中文,繁體中文（香港）,繁體中文（台灣）,Deutsch,français,Nederlands,latviešu,русский,Українська,日本語,español,português do Brasil,polski,български,italiano,suomi,Türkçe,čeština,српски,svenska,한국어", ",")
 	defaultPullRequestWorkInProgressPrefixes = strings.Split("WIP:,[WIP]", ",")
+	defaultThemes                            = strings.Split("gitea", "arc-green")
 )