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

cmd/admin.go

@@ -340,6 +340,7 @@ func runCreateUser(c *cli.Context) error {
 		IsActive:           true,
 		IsAdmin:            c.Bool("admin"),
 		MustChangePassword: changePassword,
+		Theme:              setting.UI.DefaultTheme,
 	}); err != nil {
 		return fmt.Errorf("CreateUser: %v", err)
 	}