This refactors some of the logic used for protecting default branches,
in particular Project#after_create_default_branch. The logic for this
method is moved into a separate service class. Ideally we'd get rid of
Project#after_create_default_branch entirely, but unfortunately
Project#after_import depends on it. This means it has to stick around
until we also refactor Project#after_import.

For branch protection levels we introduce
Gitlab::Access::BranchProtection, which provides a small wrapper around
Integer based branch protection levels. Using this class removes the
need for having to constantly refer to Gitlab::Access::PROTECTION_*
constants.