WIP: Autolink URLs in blobs and diffs

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

Contains three attempts/approaches at autolinking:

• 134c848c
• 86c54979
• 798d3289

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