fix(repo): unify DEFAULT_SHOW_FULL_NAME output in templates and dropdown (#36597)

The design of DefaultShowFullName has some problems, which make the UI
inconsistent, see the new comment in code

This PR does a clean up for various legacy problems, and clarify some
"user name display" behaviors.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/setting/ui.go

@@ -25,7 +25,6 @@ var UI = struct {
 	ReactionMaxUserNum      int
 	MaxDisplayFileSize      int64
 	ShowUserEmail           bool
-	DefaultShowFullName     bool
 	DefaultTheme            string
 	Themes                  []string
 	FileIconTheme           string
@@ -43,6 +42,15 @@ var UI = struct {
 
 	AmbiguousUnicodeDetection bool
 
+	// TODO: DefaultShowFullName is introduced by https://github.com/go-gitea/gitea/pull/6710
+	// But there are still many edge cases:
+	// * Many places still use "username", not respecting this setting
+	// * Many places use "Full Name" if it is not empty, cause inconsistent UI for users who have set their full name but some others don't
+	// * Even if DefaultShowFullName=false, many places still need to show the full name
+	// For most cases, either "username" or "username (Full Name)" should be used and are good enough.
+	// Only in very few cases (e.g.: unimportant lists, narrow layout), "username" or "Full Name" can be used.
+	DefaultShowFullName bool
+
 	Notification struct {
 		MinTimeout            time.Duration
 		TimeoutStep           time.Duration