Add options for sorting tables to plonematch theme
Add sortable class to Formats tables
supported-formats: make ratings more compact

This is similar to how it was on the LOCI version of the site, and is
IMO much prettier and easier to read, since the ratings take up much
less space.
Add "make plone" target to sphinx docs makefile

This builds the HTML docs with Plone theme enabled, and all warnings
flagged as errors. (In other words: -W -D html_theme=plonematch)

This target is useful for verifying more precisely how things will look
when embedded within the Plone site. For example, without the plonematch
theme enabled, the sorttable functionality does not work.
ICS writer: fix channel count correction
supported-formats: sort ratings columns properly

This necessitated the addition of alt tags on the ratings images
(unfortunately, reStructured text does not support title tags),
as well as a hack to sorttable.js to respect them.
supported-formats.txt: clarify ratings legend

It is now called "Ratings legend and definitions" so that people know
that clicking the link will also explain the column headers, too.

Thanks to Helen Flynn for the suggestion.
supported-formats.txt: add note about sorting

This adds a note (to the HTML version only) that you can sort by any of
the columns by clicking on its header.

Thanks to Helen Flynn for the suggestion.
Add sorting note to other sortable tables, too
Merge pull request #383 from ctrueden/sorttable-dev_4_4

Make format tables sortable
Merge pull request #385 from melissalinkert/ics-writer-fixes

ICS writer: fix channel count correction
Add external list of broken URLs to linkcheck_ignore