Refactor protected branch resource and spec

What does this MR do?

Some improvements to the protected branch resource and spec in preparation for adding a new one (see gitlab-org/quality/testcases#117 (closed))

Renames Resource::Branch to Resource::ProtectedBranch to be clear that it configures the settings to protect a branch, rather than just creating a new branch.

Makes branch an attribute to make it clear creating the branch is not the main responsibility of the class.

Initializes the project with a readme so that the resource only has to push once when it creates a branch.

Stops ProjectPush from visiting the project page so that it doesn't have to use the browser UI.

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/16310

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers