Fix tags view (#23243)

This PR fixes several issues reported in
https://github.com/go-gitea/gitea/issues/23221.

It does three things:
1. Fixes the `DefaultBranch` variable that has not been set.
2. Sets `Title` and `Message` for newly created tags from the Tag
message. This makes it easier to create releases from tags that have
messages and for those that don't it doesn't have any effect.
3. Makes UI changes so that tags look more like proper releases.

Before:

![2023-03-02-12-31-19](https://user-images.githubusercontent.com/1718963/222416890-941a74d4-9cd0-4c45-a59e-199d2580cd8c.png)

After:

![2023-03-02-12-31-31](https://user-images.githubusercontent.com/1718963/222416919-abce2009-8955-4cd0-9bed-1374582e04f7.png)

I purposefully didn't reformat the template so that the diff is cleaner
but can do so if that's welcome.

Thanks for your time!

---------

Signed-off-by: Wiktor Kwapisiewicz <wiktor@metacode.biz>

routers/web/repo/release.go

@@ -226,8 +226,8 @@ func releasesOrTagsFeed(ctx *context.Context, isReleasesOnly bool, formatType st
 
 // SingleRelease renders a single release's page
 func SingleRelease(ctx *context.Context) {
-	ctx.Data["Title"] = ctx.Tr("repo.release.releases")
 	ctx.Data["PageIsReleaseList"] = true
+	ctx.Data["DefaultBranch"] = ctx.Repo.Repository.DefaultBranch
 
 	writeAccess := ctx.Repo.CanWrite(unit.TypeReleases)
 	ctx.Data["CanCreateRelease"] = writeAccess && !ctx.Repo.Repository.IsArchived
@@ -241,6 +241,12 @@ func SingleRelease(ctx *context.Context) {
 		ctx.ServerError("GetReleasesByRepoID", err)
 		return
 	}
+	ctx.Data["PageIsSingleTag"] = release.IsTag
+	if release.IsTag {
+		ctx.Data["Title"] = release.TagName
+	} else {
+		ctx.Data["Title"] = release.Title
+	}
 
 	err = repo_model.GetReleaseAttachments(ctx, release)
 	if err != nil {