Enforce repository size limit across all projects and groups, includes LFS objects in that limit.

Limit can be set globally, and overridden per group, and/or project.

Backend functionality is there and comprehensive tests are included, but there is still some frontend work to be done and documentation to be added. I'm submitting early for review as we are close to the release.

@DouweM @dbalexandre I'd appreciate it if you both could start with the review while I finish the documentation and the missing frontend parts.

/cc @JobV @regisF 

Fixes #559

Replaces gitlab-org/gitlab-ce!6020

## Screenshots (see gitlab-org/gitlab-ce!6020 for more)

![Screen_Shot_2016-09-18_at_9.55.38_PM](/uploads/66eeaced1f27c7e2115feaa4775a6e99/Screen_Shot_2016-09-18_at_9.55.38_PM.png)

![Screen_Shot_2016-09-18_at_9.57.12_PM](/uploads/d811d6c184044df527bd2f81cff651ce/Screen_Shot_2016-09-18_at_9.57.12_PM.png)

![Screen_Shot_2016-09-18_at_9.58.03_PM](/uploads/a2c5b2695454dda639537304a1bcd99b/Screen_Shot_2016-09-18_at_9.58.03_PM.png)

![Screen_Shot_2016-09-19_at_1.44.19_PM](/uploads/4cc6cca7536787bde49c0b086086cbcb/Screen_Shot_2016-09-19_at_1.44.19_PM.png)

See merge request !740