Add support for PRs across forks

[czechboy0]

Currently Builda fails because it can't find the PR branch, because it lives in a fork instead of the original repository. I need to find a way to detect that case and create the bot with the correct origin URL and such.

[czechboy0]

So I have this working, I just needed to pipe through the fork's origin URL to be modified in the project's metadata. However I'm not sure if it's a valid case to have DVTSourceControlWorkspaceBlueprintRemoteRepositoriesKey be the same key for two different repo origins. Right now it seems to work to have both the original origin and the fork's origin under the same key, but more info needs to be gathered before I can call this safe. Looking into the Xcode Server's CouchDB storage should give a hint about whether something clashing between the two repo origins or not.

[czechboy0]

I took a look into xcs's couchdb and it seems like the remote repository key doesn't have to be unique, because the information is stored per-bot anyway. Technically it probably should be unique, but I can't see a case at the moment where this would cause any problems (but I'm sure something will turn up eventually 👍)