Ability to enforce Repository size limit

Description including problem, use cases, benefits, and/or goals

Projects, especially with LFS, can grow very large. Storage space is limited and if you have a large instance growth can be hard to control.

We will have a limit per project, per group and globally.

This should count total data of the project, including LFS storage size.

Users can set a limit to project size at a project level, group level and global level.
By default it's unlimited at every level.
When setting a limit at either group or global levels, we check if the group or the account has projects that are above this limit.
- If there are, we warn the user about it but they can still save this limit.
- This is a hard limit. That means we will block git pushto projects that are over the limit.
Administrators (and only them) can override both group and global limits by manually entering a value at the group level, or at the project level.

UI changes

admin/application_settings > Account and Limit settings: Add a project size limit field.
Add/Edit group: add ability to override the limit through the UI
admin/groups/<group-name>/edit, ability to change to override the global "per project limit" for this group
Error messages in the UI when there is no available space anymore:
- Unable to commit through the UI
- Unable to proceed with merging
List of projects admin/projects, if a global or goup limit is set, indicate the size of the project and the max allowed size.

Backend changes

We can't obviously put negative values for the project size limit.
By default, project size is set to unlimited.
Forks will not inherit the project limit if one is set in the original project.
New API calls to be able to override the default project size limit on a group level, and at the project level.
Error messages when doing git operations from the CLI if there is no available space anymore

List of projects: as shown in the wireframe above: next to each project in the admin view , indicate the space it takes as well as the available space (like 2.32MB out of 10Gb).
When setting a project limit globally in the admin panel, how would the warning message look like (You have 3 projects over the limit you are trying to set)?
Visual of a message that appear when we can't commit through the UI or proceed with merge (Your repository exceeds the limit of XXX and as a result you will be unable to push to this repository). I don't know if we have something for that already.

This feature will be a great addition to an upcoming CE feature: As a global admin, enable LFS per repository