Add links from changelog entries to their ticket via extlink

The changelog currently informally references an entry's ticket/issue.
* Because the changelog is now used in the Sphinx/HTML documentation hyperlinks are possible
* Because issues are now spread over two trackers (old issues in trac and new ones on github) having the same informal style will probably be confusing for readers of the documentation.

Sphinx's extlinks can be used to unambiguously link changelog entries to their issue without being significantly more verbose than the informal version (it's slightly less verbose for historical trac refs, quite a bit more for github refs). The extlink prefixes were configured to keep the final rendering similar to the original.