Refactor "htmx" to "fetch action" (#37208)

The only remaining (hard) part is "templates/repo/editor/edit.tmpl", see the FIXME

By the way:

* Make "user unfollow" use basic color but not red color, indeed it is not dangerous
* Fix "org folllow" layout (use block gap instead of inline gap)

templates/shared/user/profile_big_avatar.tmpl

@@ -115,16 +115,19 @@
 			{{end}}
 			{{if and .IsSigned (ne .SignedUserID .ContextUser.ID)}}
 				{{if not .UserBlocking}}
-				<li class="follow" hx-target="#profile-avatar-card" hx-indicator="#profile-avatar-card">
-					{{if $.IsFollowing}}
-						<button hx-post="{{.ContextUser.HomeLink}}?action=unfollow" class="ui basic red button">
+				<li class="follow">
+					{{$buttonExtraClass := Iif $.IsFollowing "" "primary"}}
+					{{$followAction := Iif $.IsFollowing "unfollow" "follow"}}
+					<button class="ui basic {{$buttonExtraClass}} button"
+									data-fetch-method="post" data-fetch-url="{{.ContextUser.HomeLink}}?action={{$followAction}}"
+									data-fetch-sync="$body #profile-avatar-card"
+					>
+						{{if $.IsFollowing}}
 							{{svg "octicon-person"}} {{ctx.Locale.Tr "user.unfollow"}}
-						</button>
-					{{else}}
-						<button hx-post="{{.ContextUser.HomeLink}}?action=follow" class="ui basic primary button">
+						{{else}}
 							{{svg "octicon-person"}} {{ctx.Locale.Tr "user.follow"}}
-						</button>
-					{{end}}
+						{{end}}
+					</button>
 				</li>
 				{{end}}
 				<li>