Add support for self-hosted GitHub

[avikalpg]

Many companies self-host their GitHub.
To run the skyline in that case, the user has to clone the repository, change the GitHub API endpoint and update the access token in order to see their skyline.

Expected behaviour

There should be a settings button on the top right, which opens a modal that asks for the GitHub API endpoint and the access token.
If the user has filled those fields, then Skyline should start working for their self-hosted GitHub as well.