Hosts using a non-standard configuration may have a different
hostname/port/scheme than what may be configured on the GitLab
server. While expose_url should generate a proper URL, there are cases
where it may not work. Since we don't need the full URL, we can use the
relative path.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/61280

Refactor api events class to use external helper

Move specs from old class

Add changelog and magic string

Refactor events class to be more explicit

Remove blank line

This reverts commit a5378665a1dc0b9c8dc3a4fa279a0eb78aac5aac.

Exceptions are no longer raised, instead all errors encountered are
added to the errors property.

MergeRequests::BuildService is used to generate attributes of a new
merge request.

Code moved from Api::Internal to Api::Helpers::InternalHelpers.

To create a new merge request:

  git push -u origin -o merge_request.create

To create a new merge request setting target branch:

  git push -u origin -o merge_request.create \
    -o merge_request.target=123

To update an existing merge request with a new target branch:

  git push -u origin -o merge_request.target=123

A new Gitlab::PushOptions class handles parsing and validating the push
options array. This can be the start of the standard of GitLab accepting
push options that follow namespacing rules. Rules are discussed in issue
https://gitlab.com/gitlab-org/gitlab-ce/issues/43263.

E.g. these push options:

  -o merge_request.create -o merge_request.target=123

Become parsed as:

  {
    merge_request: {
      create: true,
      target: '123',
    }
  }

And are fetched with the class via:

  push_options.get(:merge_request)
  push_options.get(:merge_request, :create)
  push_options.get(:merge_request, :target)

A new MergeRequests::PushOptionsHandlerService takes the `merge_request`
namespaced push options and handles creating and updating
merge requests.

Any errors encountered are passed to the existing `output` Hash in
Api::Internal's `post_receive` endpoint, and passed to gitlab-shell
where they're output to the user.

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/43263

This reverts merge request !26823

Allow multiple repositories per project

See merge request gitlab-org/gitlab-ce!26539

This backports various parameter changes EE makes to API::Projects,
along with moving some code around to make it easier to extend in EE.