Support github enterprise in version 2.0 codebase

[danielpops]

Describe what problem your feature request solves The github oauth configuration assumes that you're using github.com and doesn't work with github enterprise.

Describe the solution you'd like A configuration in .env (or equivalent for prod) that lets you specify a github enterprise url

Additional context #250 was originally created and implemented in #252 but testing was inconclusive and the changes never merged. @jgadsden suggested to implement the feature in the version 2.0 codebase instead to leverage easier development and debuggability.

[danielpops]

Hi @lreading, just wanted to check back in on this. Any thoughts on if / when this might be revisited?

I'd love to use Threatdragon to maintain our company's threat models and store them in our internal github enterprise repo(s)!

[lreading]

I've been less active recently, my apologies! Hoping to get more time onto this specific issue in the next month or so.

[jgadsden]

@danielpops and @lreading , this may not make in into version 2.0, so labelling it for version 2.x

[jgadsden]

Unlikely to get this into the imminent version 2.0 release, so pushing it back to version 2.x