Avoid database lookups for `DescriptionHTML` (#18924)

* Avoid database lookups for `DescriptionHTML`

- Don't Compose meta's for DescriptionHTML, they are only needed in
order to correctly format and show issue's but it's highly unlikely that
a repository description will refer to a local issue.

Using 125 Connections for 5 seconds: on `/explore/repos`(which is the most
noticeable usage by this function's database lookups):

Before:

Statistics        Avg      Stdev        Max
  Reqs/sec       569.41     506.05    2715.00
  Latency      214.27ms    16.60ms   294.84ms
  HTTP codes:
    1xx - 0, 2xx - 2974, 3xx - 0, 4xx - 0, 5xx - 0
    others - 0
  Throughput:    27.17MB/s

After:

Statistics        Avg      Stdev        Max
  Reqs/sec      1585.04     789.84    4144.56
  Latency       78.89ms    15.89ms   206.94ms
  HTTP codes:
    1xx - 0, 2xx - 7975, 3xx - 0, 4xx - 0, 5xx - 0
    others - 0
  Throughput:    73.85MB/s

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: zeripath <art27@cantab.net>

models/repo/repo.go

@@ -520,7 +520,7 @@ func (repo *Repository) DescriptionHTML(ctx context.Context) template.HTML {
 	desc, err := markup.RenderDescriptionHTML(&markup.RenderContext{
 		Ctx:       ctx,
 		URLPrefix: repo.HTMLURL(),
-		Metas:     repo.ComposeMetas(),
+		// Don't use Metas to speedup requests
 	}, repo.Description)
 	if err != nil {
 		log.Error("Failed to render description for %s (ID: %d): %v", repo.Name, repo.ID, err)