Add links from changelog entries to their ticket via extlink

Supports links to the new github (``:gh:``) and the old trac
(``:trac:``). The prefixes are used to keep the same reference style
as pre-extlinks in the final output (aside from a few entries at the
bottom of 1.0, trac references were all in the form ``ticket #42``).

--HG--
extra : amend_source : fc44e96e503ce83648b04cdd4452dbecbb27c028