Emoji approve

[FlorianLudwig]

First few bits needed for handling emoji to approve merge requests. Includes first steps for #146 and #145.

A few ideas on how to improve upon this:

1. approver eligability

I think it would be best to implement this on CODEOWNERS file in the repo. A quicker implementation might be to add this to the .marge-bot.yml:

approvers: # List of gitlab user names that are allowed to approve MR
 - joe_d
 - max_m

(which I did first but I prefer the CODEOWNERS solution so I did not publish it)

2. allow overwriting of all configs in .marge-bot.yml

As stated in #146 it would be great to overwrite the global config. But that would go in a separate merge request.

3. use marge-bot.yml from destination of merge request instead

Currently it uses the .marge-bot.yml config hardcoded from master, probably it would be better to use it from the merge request destination tree. Objections?

4. handle down votes

My first implementation did saw any 👎 as blocker not accepting a merge request as long as there is a single 👎. As that might not be sensible to do in every environment I decided to remove handling of 👎 at all for the merge request.

I think there might be three different ways to handle this:

# in any case only count thumbs of approvers that are configured to be allowed to do so.
approver_count_1 = thumbs_up - thumbs_down
approver_count_2 = thumbs_up if not thumbs_down else 0
approver_count_3 = thumbs_up # and ignore thumbs_down

It is a somewhat tricky topic so I guess it got to be configurable... :]