WIP: Publish handbook as epub

This is actually a follow-up to @nick.thomas's MR !2940 (closed).

This adds a rake task to build a single epub file of the GitLab Handbook. This can be installed on any e-reader.

How it works:

1. All .md of the handbook are joined in one big all-in-one markdown file.
2. During the process links are adjusted so conflicts between anchors do not occur.
3. The all-in-one markdown is converted by middleman
4. The single html page created by middleman is converted to epub by pandoc.

It might feel like a very cumbersome method, but this is the best method I could find.

@nick.thomas mentioned some tasks in his MR:

• Missing images like https://github.com/jgm/pandoc/issues/1450
• Pandoc doesn't support all markdown extensions we use
• Needs a funky cover image
• Many internal links don't work. (EDIT: This might need some double checking)
• Many warnings like this in the generated output

But there also some other things that need to figured out:

• Make it work side-to-side with the regular handbook webpage. (at the moment that one is deleted)
• Code is really dirty and hard to maintain, fix that. I am really open on suggestions on how to improve that.
• Title and some other attributes are not set yet.
• Undo the hack for the team/structure proxy.
• Add it to CI and make it downloadable.
• optional: Also build a pdf.

At the moment this is the epub that rolls out: handbook.epub

I don't think it is good, but it should be complete and be actual usable.

Related issues

#689