Support for source import API

[itsmemattchung]

Support Import API

• [ ] Start an import
• [ ] Get import progress
• [ ] Update existing import
• [ ] Get commit authors
• [ ] Map a commit author
• [ ] Set Git LFS preference
• [ ] Get large files
• [ ] Cancel an import

I'll begin with "starting the import".

--- Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/34013392-support-for-source-import-api?utm_campaign=plugin&utm_content=tracker%2F183477&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F183477&utm_medium=issues&utm_source=github).