Automatically determine edit_link default

[mortenpi]

Generalizes the solution to devbranch to also apply to edit_link. Fix #1827.

It also now prints a warning (for both devbranch and edit_link) if the git remote solution somehow fails. I think this is a good idea, because otherwise you might get some weird intermittent environment-dependent failures which are hard to debug. The warning should at least give you a hint where things might be going wrong.

[fredrikekre]

I noticed in Literate that the call to git remote show here takes ~1s (fredrikekre/Literate.jl@8054d26). It is not a huge problem here since you only call makedocs once, but a bit annoying when using LiveServer for example. (It wasn't a problem before since it was only used in deploydocs which doesn't run in that case).

[mortenpi]

Should we also maybe do #1838 for this?

That said, something that I didn't think about before, I believe this could potentially block for up to 300s (TCP timeout) if you're having some weird networking problems.