WIP: Autolink URLs in blobs and diffs

Open Douwe Maan requested to merge dm-autolink-in-blobs into master

Extracted from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4121.

Contains three attempts/approaches at autolinking:

None of these have acceptable performance.

To do

  • Make sure blob rendering performance doesn't suffer dramatically
  • Don't run pre-processing pipeline, because this may break the highlighting