Clarify "text-align" CSS helpers, fix clone button padding (#25763)

Changes:

* Rename gt-tl/gt-tc/gt-tr to gt-text-left/gt-text-center/gt-text-right
* The gt-ab and gt-br-0 are removed because they are not needed anymore
* Fix the clone dropdown button padding by ":not(.icon)"

Before:

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/eb030633-622c-4ca7-8e88-ce010d9f51a6)

</details>

After:

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/64c09403-bf21-439c-88f1-780b34ccab6b)

</details>

Fixes #25758

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

templates/repo/header.tmpl

@@ -121,7 +121,7 @@
 								<div class="header">
 									{{$.locale.Tr "repo.already_forked" .Name}}
 								</div>
-								<div class="content gt-tl">
+								<div class="content gt-text-left">
 									<div class="ui list">
 										{{range $.UserAndOrgForks}}
 											<div class="ui item gt-py-3">