Feature/mr labels

## What does this MR do?
Implements labels for merge requests. It unifies the labels between issues and merge requests - if a label is added to an issue that same label will be visible in the merge requests tab as well. There's enough similarity between MRs and issues I didn't think it made sense to separate the labels.

## Are there points in the code the reviewer needs to double check?
See code comments.

## Why was this MR needed?
Labels have been available on issues for some time. It would really be nice to have them available for MRs, too. It's also been a feature request for about a year.

## What are the relevant issue numbers / Feature requests?
http://feedback.gitlab.com/forums/176466-general/suggestions/3958540-add-way-to-label-merge-requests

## Screenshots
![screen shot 2014-04-14 at 9 11 17 pm](https://cloud.githubusercontent.com/assets/2394772/2703030/cc3b0092-c443-11e3-9252-45fa9c175f11.png)

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![screen shot 2014-04-14 at 9 10 43 pm](https://cloud.githubusercontent.com/assets/2394772/2703031/cc3b00b0-c443-11e3-8b3a-9ea5d6d0ecc5.png)

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![screen shot 2014-04-14 at 9 11 27 pm](https://cloud.githubusercontent.com/assets/2394772/2703032/cc3c6afe-c443-11e3-9610-432ce414c803.png)