Add the ability to have built in themes in Gitea (#4198)

This makes it easier for user who want to theme but
don't have the ability to know how to customize templates
all that is required is a change in a config option

The reason why I chose the DEFAULT_THEME as variable,
as perhaps in the future we will allow users to chose their
theme whon logged in just like we do with languages

modules/setting/setting.go

@@ -280,6 +280,7 @@ var (
 		ThemeColorMetaTag   string
 		MaxDisplayFileSize  int64
 		ShowUserEmail       bool
+		DefaultTheme        string
 
 		Admin struct {
 			UserPagingNum   int
@@ -303,6 +304,7 @@ var (
 		ReactionMaxUserNum:  10,
 		ThemeColorMetaTag:   `#6cc644`,
 		MaxDisplayFileSize:  8388608,
+		DefaultTheme:        `gitea`,
 		Admin: struct {
 			UserPagingNum   int
 			RepoPagingNum   int