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

models/migrations/v77.go

@@ -0,0 +1,17 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migrations
+
+import (
+	"github.com/go-xorm/xorm"
+)
+
+func addUserDefaultTheme(x *xorm.Engine) error {
+	type User struct {
+		Theme string `xorm:"VARCHAR(30)"`
+	}
+
+	return x.Sync2(new(User))
+}