Support wildcard matches for protected branches at the model level.

1. The main implementation is in the `ProtectedBranch` model. The
   wildcard is converted to a Regex and compared. This has been tested
   thoroughly.

    - While `Project#protected_branch?` is the main entry point,
      `project#open_branches` and
      `project#developers_can_push_to_protected_branch?`
      have also been modified to work with wildcard protected branches.

    - The regex is memoized (within the `ProtectedBranch` instance)

2. Improve the performance of `Project#protected_branch?`

    -  This method is called from `Project#open_branches` once _per branch_
       in the project, to check if that branch is protected or not.

    -  Before, `#protected_branch?` was making a database call every
       time it was invoked (in the above case, that amounts to once
       per branch), which is expensive.

    -  This commit caches the list of protected branches in memory, which
       reduces the number of database calls down to 1.

    -  A downside to this approach is that `#protected_branch?` _could_
       return a stale value (due to the caching), but this is
       an acceptable tradeoff.

3. Remove the (now) unused `Project#protected_branch_names` method.

    - This was previously used to check for protected branch status.

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