Merge user branch into the master branch

[mandolyte]

DoD
Given a pull request against a user branch with no conflicts with the master branch, enable the administrator to merge the user branch to the master branch.

[mandolyte]

The merge feature is depends on having a pull request for the user branch. So the workflow and UI/X will be something like this:

• translator informs administrator that their work is ready
• the translator could send send the admin the CV spreadsheet for their branch as substantiation of the request to merge
• the admin will go to the merge branch page in gateway Admin
• the admin will select the repo/resource
• all the branches in that repo will be shown and the admin will select one
• the admin will click the "merge" button
• the app will then create the PR using create pull request api
• if the create PR API call succeeds, then proceed. Otherwise show the error.
• if the PR response indicates that the branch has conflicts, then inform the user that it must be done manually. Otherwise, continue.
• use the api to merge
• if merge succeeds, inform the admin; otherwise show error message.