1. Remove the `developers_can_push` and `developers_can_merge` boolean
columns.
2. Add two new tables, `protected_branches_push_access`, and
`protected_branches_merge_access`. Each row of these 'access' tables is
linked to a protected branch, and uses a `access_level` column to
figure out settings for the protected branch.
3. The `access_level` column is intended to be used with rails' `enum`,
with `:masters` at index 0 and `:developers` at index 1.
4. Doing it this way has a few advantages:
- Cleaner path to planned EE features where a protected branch is
accessible only by certain users or groups.
- Rails' `enum` doesn't allow a declaration like this due to the
duplicates. This approach doesn't have this problem.
enum can_be_pushed_by: [:masters, :developers]
enum can_be_merged_by: [:masters, :developers]
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