Rework header bar on issue, pull requests and milestone (#24420)

- Make search bar dynamic full width via flexbox
- Make all buttons `small` so font size is the same for all elements in
the header
- Remove primary color from search field, add SVG icon like on Code tab
- Fix button vertical padding being enlarged by SVG icons

[View diff without
whitespace](https://github.com/go-gitea/gitea/pull/24420/files?diff=unified&w=1)

<img width="1226" alt="Screenshot 2023-04-29 at 11 58 53"
src="https://user-images.githubusercontent.com/115237/235296851-74848267-664f-4c1f-b94c-a1b94196ff75.png">
<img width="1219" alt="Screenshot 2023-04-29 at 11 59 39"
src="https://user-images.githubusercontent.com/115237/235296852-bcfde5ed-8658-43c2-b7e5-3ad84611e76f.png">

Mobile:
<img width="437" alt="Screenshot 2023-04-29 at 11 59 52"
src="https://user-images.githubusercontent.com/115237/235296860-99263373-7b27-4540-868c-a93e70f281ca.png">
<img width="433" alt="Screenshot 2023-04-29 at 12 00 00"
src="https://user-images.githubusercontent.com/115237/235296862-6cf64317-a864-405a-a00f-b5ab620349f5.png">

templates/projects/list.tmpl

@@ -3,14 +3,14 @@
 		{{if .CanWriteProjects}}
 			<div class="navbar">
 				<div class="ui right">
-					<a class="ui green button" href="{{$.Link}}/new">{{.locale.Tr "repo.projects.new"}}</a>
+					<a class="ui small green button" href="{{$.Link}}/new">{{.locale.Tr "repo.projects.new"}}</a>
 				</div>
 			</div>
 			<div class="ui divider"></div>
 		{{end}}
 
 		{{template "base/alert" .}}
-		<div class="ui compact tiny menu">
+		<div class="small-pill-buttons ui compact tiny menu">
 			<a class="item{{if not .IsShowClosed}} active{{end}}" href="{{$.Link}}?state=open">
 				{{svg "octicon-project-symlink" 16 "gt-mr-3"}}
 				{{.locale.PrettyNumber .OpenCount}}&nbsp;{{.locale.Tr "repo.issues.open_title"}}

templates/projects/new.tmpl

@@ -3,7 +3,7 @@
 		<div class="navbar">
 			{{if and .CanWriteProjects .PageIsEditProject}}
 			<div class="ui right floated secondary menu">
-				<a class="ui green button" href="{{$.HomeLink}}/-/projects/new">{{.locale.Tr "repo.milestones.new"}}</a>
+				<a class="ui small green button" href="{{$.HomeLink}}/-/projects/new">{{.locale.Tr "repo.milestones.new"}}</a>
 			</div>
 			{{end}}
 		</div>

templates/projects/view.tmpl

@@ -5,7 +5,7 @@
 			</div>
 			<div class="column right aligned">
 				{{if .CanWriteProjects}}
-					<a class="ui green button show-modal item" data-modal="#new-board-item">{{.locale.Tr "new_project_column"}}</a>
+					<a class="ui small green button show-modal item" data-modal="#new-board-item">{{.locale.Tr "new_project_column"}}</a>
 				{{end}}
 				<div class="ui small modal new-board-modal" id="new-board-item">
 					<div class="header">