Refactor the repo/remote handling

[mortenpi]

This factors out the refactoring part of #1254 to make the whole thing a little more manageable. In short, it introduces the Remotes module which allows for a more flexible way to define the remote repository. The Remote objects can be passed via the repo keyword to makedocs:

makedocs(repo = Remotes.GitHub("JuliaDocs", "Documenter.jl"), ...)

The user can also define a custom Remotes.Remote type and overload the Remotes.repourl and Remotes.fileurl functions to control the link generation.

We do quite a bit of special handling of GitLab, BitBucket and Azure Devops repositories, so it would probably be worth adding special Remotes.Remote types for each of those too.

[mortenpi]

I think this in a mergeable shape now. I reduced it down to the bare minimum to introduce the Remotes module, without changing any behavior as far as I can tell (except one case, where I replaced one buggy behavior with another).

I do have a bunch of follow-up ideas, to properly fix up getremote etc., which will likely change some behaviors and also refactor parts of the code. And then there is the UI change in #1254. But to make it manageable to review and to make sure I am not accidentally changing or breaking anything I did not intend, I want to merge this minimal version first and do those things in follow-up PR(s).

@fredrikekre Would you have time to look through this? It's a little big, but I would love to get another set of eyes on this.

[mortenpi]

Thanks @fredrikekre!