Remove delay when hitting Reply... button on page with a lot of comments

Every time the "Reply..." button was hit, a new `GLForm` was instantiated (which is fine), which would then call `GitLab.GfmAutoComplete.setup()` without specifying which textarea we needed autocompletion on, which resulted in `GitLab.GfmAutoComplete.setup` iterating through every single textarea on the page. On a page with a lot of comments, where each comment would have its own textarea that will be used to edit the comment, this would take a long time and cause the whole page to hang for 2 seconds when a  "Reply..." button was clicked.

See merge request !5602