Create a protected branches API

Currently, the only way to protect/unprotect a branch via the API is to use the branches API
The branches API does not mirror the internal data model for protected branches - this can't be changed due to backward compatibility
This causes a number of edge cases in the code that glues these two data models together

Create a new "protected_branches" API that allows performing CRUD actions directly on protected branches, rather than operating on git branches
The API should allow performing all actions that are currently possible through the web interface:
- Create a wildcard protected branch
- Set the "Allowed to push" setting to "Developers", "No one", or "Developers + Masters"
- Set the "Allowed to push" setting to any combination of the above, in addition to specific users and groups (EE-only)
- Set the "Allowed to merge" setting to "Developers" or "Developers + Masters"
- Set the "Allowed to merge" setting to any combination of the above, in addition to specific users and groups (EE-only)