added endpoints for branch / pr / deployment summary

[bradrydzewski]

This pull request adds new database capabilities to index the latest build for each branch, tag, pull request and deployment for efficient retrieval. It also updates our webhook code to process branch delete hooks and pull request close hooks to remove from the index.

The latest build data per branch and pull request is exposed via the below endpoints. We also added endpoints to remove an entry from the index.

GET    /api/repos/{namespace}/{name}/builds/branches
DELETE /api/repos/{namespace}/{name}/builds/branches/{branch}
GET    /api/repos/{namespace}/{name}/builds/pulls
DELETE /api/repos/{namespace}/{name}/builds/pulls/{pr}

The purpose of these endpoints is to facilitate screens in the user interface that display the latest branch, pull request and deployment details. Example deployment screen:

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