Replace links to Trac on Sage website, wiki, documentation

[mkoeppe]

[mezzarobba]

Until this is done, I suppose we can still use :trac: links in doc to refer to github issues?

[mkoeppe]

I think we can continue to use the :trac: Sphinx role, but we should change its definition so it points to GH Issues instead.

Some care needs to be taken because :trac: is also used for referring to some Trac wiki pages. These need to be taken of separately, case by case, because the page titles may have been mangled.

[kwankyu]

Nonetheless, shouldn't we add a new :gh: role to take the role of :trac: at the same time?

[mkoeppe]

Yes, I think that's a good idea.