Ask for Bitbucket username during installation

[michaelsauter]

Fixes #652.

Also, the Bitbucket user actually does not need admin permissions, write is enough.

Tasks:

• [x] Updated design documents in docs/design directory or not applicable
• [x] Updated user-facing documentation in docs directory or not applicable
• [x] Ran tests (e.g. make test) or not applicable
• [x] Updated changelog or not applicable