This is a version of gh-4993 for the `3.x-stable` branch.
The GitHub UI treats `#NUMBER` as referring to its own issues which is confusing
when in jQuery source it's usually referring to the old deprecated Trac instance
at https://bugs.jquery.com. This change replaces all such Trac references with
`trac-NUMBER`.
A few of the references came with the Sizzle integration and referred to the
Sizzle GitHub bug tracker. Those have been replaced with full links instead.
A new entry describing issue reference conventions has been added to README.
Closes gh-4994
Ref gh-4993
Ref 5d5ea01511
