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/setting.go

@@ -33,9 +33,9 @@ import (
 	"github.com/go-macaron/session"
 	_ "github.com/go-macaron/session/redis" // redis plugin for store session
 	"github.com/go-xorm/core"
-	"github.com/kballard/go-shellquote"
-	"github.com/mcuadros/go-version"
-	"gopkg.in/ini.v1"
+	shellquote "github.com/kballard/go-shellquote"
+	version "github.com/mcuadros/go-version"
+	ini "gopkg.in/ini.v1"
 	"strk.kbt.io/projects/go/libravatar"
 )
 
@@ -303,6 +303,7 @@ var (
 		MaxDisplayFileSize  int64
 		ShowUserEmail       bool
 		DefaultTheme        string
+		Themes              []string
 
 		Admin struct {
 			UserPagingNum   int
@@ -329,6 +330,7 @@ var (
 		ThemeColorMetaTag:   `#6cc644`,
 		MaxDisplayFileSize:  8388608,
 		DefaultTheme:        `gitea`,
+		Themes:              []string{`gitea`, `arc-green`},
 		Admin: struct {
 			UserPagingNum   int
 			RepoPagingNum   int