Clean up polluted styles and remove dead CSS code (#24497)

Follow #24393


The funny history:

* At the beginning, `.ui.message` was polluted by `text-align: center`
* Then people do `<div class="ui ... message text left">`
* But `.ui.left` is polluted by `float: left`
* Then people do `#xxx .ui.message { width: 100% !important;}`

The code just becomes more and more hacky.

After removing the pollution,  everything becomes clear and straight.


And, this PR also does:

1. Remove the `package.css`, its styles could be provided by `top
aligned`
2. Remove `#avatar-arrow`, dead code



Screenshot:


![image](https://user-images.githubusercontent.com/2114189/235862130-a9eb5d8f-7d01-457c-99f7-21d0abc3075e.png)


![image](https://user-images.githubusercontent.com/2114189/235862222-139709a7-95c2-4f89-a40f-100b2d76d9bb.png)

Co-authored-by: Giteabot <teabot@gitea.io>

templates/base/alert.tmpl

@@ -1,20 +1,20 @@
 {{if .Flash.ErrorMsg}}
-	<div class="ui negative message flash-error">
+	<div class="ui negative message flash-message flash-error">
 		<p>{{.Flash.ErrorMsg | Str2html}}</p>
 	</div>
 {{end}}
 {{if .Flash.SuccessMsg}}
-	<div class="ui positive message flash-success">
+	<div class="ui positive message flash-message flash-success">
 		<p>{{.Flash.SuccessMsg | Str2html}}</p>
 	</div>
 {{end}}
 {{if .Flash.InfoMsg}}
-	<div class="ui info message flash-info">
+	<div class="ui info message flash-message flash-info">
 		<p>{{.Flash.InfoMsg | Str2html}}</p>
 	</div>
 {{end}}
 {{if .Flash.WarningMsg}}
-	<div class="ui warning message flash-warning">
+	<div class="ui warning message flash-message flash-warning">
 		<p>{{.Flash.WarningMsg | Str2html}}</p>
 	</div>
 {{end}}