Clean up diff header css and reduce global textarea min-height (#29232)

1. Tweak diff header and remove a numbe of unneeded CSS for it:

Before:
<img width="433" alt="Screenshot 2024-02-18 at 01 08 09"
src="https://github.com/go-gitea/gitea/assets/115237/d8b377c0-57bc-44d5-bb57-a582c7d4b3b4">

After:
<img width="463" alt="Screenshot 2024-02-18 at 01 07 56"
src="https://github.com/go-gitea/gitea/assets/115237/d08c17e7-5b86-4d07-81da-6371f4754325">

3. Reduce height of review textarea and also reduce fomantic's CSS from
12em to 8em. Now fits better on my screen:

<img width="1352" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/5c658d13-295e-4929-94da-13ade888020d">

---------

Co-authored-by: delvh <dev.lh@web.de>

templates/repo/commit_page.tmpl

@@ -184,7 +184,7 @@
 				</div>
 		</div>
 		{{if .Commit.Signature}}
-			<div class="ui bottom attached message gt-text-left gt-df gt-ac gt-sb commit-header-row gt-fw {{$class}}">
+			<div class="ui bottom attached message gt-text-left gt-df gt-ac gt-sb commit-header-row gt-fw gt-mb-0 {{$class}}">
 				<div class="gt-df gt-ac">
 					{{if .Verification.Verified}}
 						{{if ne .Verification.SigningUser.ID 0}}

templates/repo/diff/box.tmpl

@@ -1,7 +1,7 @@
 {{$showFileTree := (and (not .DiffNotAvailable) (gt .Diff.NumFiles 1))}}
 <div>
 	<div class="diff-detail-box diff-box">
-		<div class="gt-df gt-ac gt-fw">
+		<div class="gt-df gt-ac gt-fw gt-gap-3 gt-ml-1">
 			{{if $showFileTree}}
 				<button class="diff-toggle-file-tree-button not-mobile btn interact-fg" data-show-text="{{ctx.Locale.Tr "repo.diff.show_file_tree"}}" data-hide-text="{{ctx.Locale.Tr "repo.diff.hide_file_tree"}}">
 					{{/* the icon meaning is reversed here, "octicon-sidebar-collapse" means show the file tree */}}