Add support for Bitbucket

[avikalpg]

Currently, skyline only works for GitHub. But it should also work for other providers - starting with Bitbucket.

Features:

1. Create a simple dropdown on the landing page for selecting the provider.
2. Add support for Bitbucket in the getUserContributions function
3. Make sure that we also support the "Bitbucket server" edition to support companies that self-host their Bitbucket. (similar to how we handle self-hosted GitHub in Add support for self-hosted GitHub #41 )