Add the ability to pin Issues (#24406)

This adds the ability to pin important Issues and Pull Requests. You can
also move pinned Issues around to change their Position. Resolves #2175.

## Screenshots

![grafik](https://user-images.githubusercontent.com/15185051/235123207-0aa39869-bb48-45c3-abe2-ba1e836046ec.png)

![grafik](https://user-images.githubusercontent.com/15185051/235123297-152a16ea-a857-451d-9a42-61f2cd54dd75.png)

![grafik](https://user-images.githubusercontent.com/15185051/235640782-cbfe25ec-6254-479a-a3de-133e585d7a2d.png)

The Design was mostly copied from the Projects Board.

## Implementation
This uses a new `pin_order` Column in the `issue` table. If the value is
set to 0, the Issue is not pinned. If it's set to a bigger value, the
value is the Position. 1 means it's the first pinned Issue, 2 means it's
the second one etc. This is dived into Issues and Pull requests for each
Repo.

## TODO
- [x] You can currently pin as many Issues as you want. Maybe we should
add a Limit, which is configurable. GitHub uses 3, but I prefer 6, as
this is better for bigger Projects, but I'm open for suggestions.
- [x] Pin and Unpin events need to be added to the Issue history.
- [x] Tests
- [x] Migration

**The feature itself is currently fully working, so tester who may find
weird edge cases are very welcome!**

---------

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

templates/repo/issue/view_content/sidebar.tmpl

@@ -530,17 +530,31 @@
 
 		{{if and .IsRepoAdmin (not .Repository.IsArchived)}}
 			<div class="ui divider"></div>
-			<div class="ui watching">
-				<button class="fluid ui show-modal button {{if .Issue.IsLocked}} negative {{end}}" data-modal="#lock">
-					{{if .Issue.IsLocked}}
-						{{svg "octicon-key"}}
-						{{.locale.Tr "repo.issues.unlock"}}
-					{{else}}
-						{{svg "octicon-lock"}}
-						{{.locale.Tr "repo.issues.lock"}}
-					{{end}}
-				</button>
-			</div>
+
+			{{if or .PinEnabled .Issue.IsPinned}}
+				<form class="gt-mt-2" method="POST" {{if $.NewPinAllowed}}action="{{.Issue.Link}}/pin"{{else}}data-tooltip-content="{{.locale.Tr "repo.issues.max_pinned"}}"{{end}}>
+					{{$.CsrfTokenHtml}}
+					<button class="fluid ui button gt-df gt-jc {{if not $.NewPinAllowed}}disabled{{end}}">
+						{{if not .Issue.IsPinned}}
+							{{svg "octicon-pin" 16 "gt-mr-3"}}
+							{{.locale.Tr "pin"}}
+						{{else}}
+							{{svg "octicon-pin-slash" 16 "gt-mr-3"}}
+							{{.locale.Tr "unpin"}}
+						{{end}}
+					</button>
+				</form>
+			{{end}}
+
+			<button class="gt-mt-2 fluid ui show-modal button {{if .Issue.IsLocked}} negative {{end}}" data-modal="#lock">
+				{{if .Issue.IsLocked}}
+					{{svg "octicon-key"}}
+					{{.locale.Tr "repo.issues.unlock"}}
+				{{else}}
+					{{svg "octicon-lock"}}
+					{{.locale.Tr "repo.issues.lock"}}
+				{{end}}
+			</button>
 			<div class="ui tiny modal" id="lock">
 				<div class="header">
 					{{if .Issue.IsLocked}}
@@ -605,7 +619,7 @@
 					</form>
 				</div>
 			</div>
-			<button class="fluid ui show-modal button negative gt-mt-3" data-modal="#delete">
+			<button class="gt-mt-2 fluid ui show-modal button negative" data-modal="#delete">
 				{{svg "octicon-trash"}}
 				{{.locale.Tr "repo.issues.delete"}}
 			</button>