1. Remove `master_or_greater?` and `developer_or_greater?` in favor of
`max_member_access`, which is a lot nicer.
2. Remove a number of instances of `include Gitlab::Database::MigrationHelpers`
in migrations that don't need this module. Also remove comments where
not necessary.
3. Remove duplicate entry in CHANGELOG.
4. Move `ProtectedBranchAccessSelect` from Coffeescript to ES6.
5. Split the `set_access_levels!` method in two - one each for `merge` and
`push` access levels.
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