fix: add missing checks to several API and web handlers (#39501) (#39507)

Backport #39501 by @silverwind

Several handlers skipped checks that their sibling routes or settings
already enforce. This brings them in line.

- Push mirror API honors `DISABLE_NEW_PUSH` and checks the caller's
permission
- Media API serves small files with the usual content headers
- Issue attachment API ignores comment attachments
- Push-to-create respects `FORCE_PRIVATE`
- Profile feeds and follow actions respect `ENABLE_FEED` and owner
visibility
- Tag delete route refuses release tags
- Refresh token grant only accepts refresh tokens
- Gitea migrations bound the source's page size

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: bircni <bircni@icloud.com>

routers/web/user/home.go

@@ -734,18 +734,10 @@ func UsernameSubRoute(ctx *context.Context) {
 			ShowGPGKeys(ctx)
 		}
 	case strings.HasSuffix(username, ".rss"):
-		if !setting.Other.EnableFeed {
-			ctx.HTTPError(http.StatusNotFound)
-			return
-		}
 		if reloadParam(".rss") {
 			feed.ShowUserFeedRSS(ctx)
 		}
 	case strings.HasSuffix(username, ".atom"):
-		if !setting.Other.EnableFeed {
-			ctx.HTTPError(http.StatusNotFound)
-			return
-		}
 		if reloadParam(".atom") {
 			feed.ShowUserFeedAtom(ctx)
 		}