Prevent re-review and dismiss review actions on closed and merged PRs (#30065)

Resolves #29965.

---
Manually tested this by:
- Following the
[installation](https://docs.gitea.com/next/installation/install-with-docker#basics)
guide (but built a local Docker image instead)
- Creating 2 users, one who is the `Owner` of a newly-created repository
and the other a `Collaborator`
- Had the `Collaborator` create a PR that the `Owner` reviews
- `Collaborator` resolves conversation and `Owner` merges PR

And with this change we see that we can no longer see re-request review
button for the `Owner`:

<img width="1351" alt="Screenshot 2024-03-25 at 12 39 18 AM"
src="https://github.com/go-gitea/gitea/assets/60799661/bcd9c579-3cf7-474f-a51e-b436fe1a39a4">

templates/repo/issue/view_content/sidebar.tmpl

@@ -59,7 +59,7 @@
 							{{end}}
 						</div>
 						<div class="tw-flex tw-items-center tw-gap-2">
-							{{if (and $.Permission.IsAdmin (or (eq .Review.Type 1) (eq .Review.Type 3)) (not $.Issue.IsClosed))}}
+							{{if (and $.Permission.IsAdmin (or (eq .Review.Type 1) (eq .Review.Type 3)) (not $.Issue.IsClosed) (not $.Issue.PullRequest.HasMerged))}}
 								<a href="#" class="ui muted icon tw-flex tw-items-center show-modal" data-tooltip-content="{{ctx.Locale.Tr "repo.issues.dismiss_review"}}" data-modal="#dismiss-review-modal-{{.Review.ID}}">
 									{{svg "octicon-x" 20}}
 								</a>
@@ -91,7 +91,7 @@
 									{{svg "octicon-hourglass" 16}}
 								</span>
 							{{end}}
-							{{if .CanChange}}
+							{{if and .CanChange (or .Checked (and (not $.Issue.IsClosed) (not $.Issue.PullRequest.HasMerged)))}}
 								<a href="#" class="ui muted icon re-request-review{{if .Checked}} checked{{end}}" data-tooltip-content="{{if .Checked}}{{ctx.Locale.Tr "repo.issues.remove_request_review"}}{{else}}{{ctx.Locale.Tr "repo.issues.re_request_review"}}{{end}}" data-issue-id="{{$.Issue.ID}}" data-id="{{.ItemID}}" data-update-url="{{$.RepoLink}}/issues/request_review">{{if .Checked}}{{svg "octicon-trash"}}{{else}}{{svg "octicon-sync"}}{{end}}</a>
 							{{end}}
 							{{svg (printf "octicon-%s" .Review.Type.Icon) 16 (printf "text %s" (.Review.HTMLTypeColorName))}}

templates/swagger/v1_json.tmpl

@@ -11276,6 +11276,9 @@
           "204": {
             "$ref": "#/responses/empty"
           },
+          "403": {
+            "$ref": "#/responses/forbidden"
+          },
           "404": {
             "$ref": "#/responses/notFound"
           },