[Video]: Renamed column 'url' as 'html'

The column will store the <video> tag as returned by SublimeVideo
Extracted out code for TeX rendering into another method ...

This is a basic operation that we will need separately when
creating <a> for videos. These <a>s - lesson titles in reality -
should be able to show mathematical eqns using TeX
Removed obsolete styling specifications
Placeholders added in Video for SublimeVideo integration ...

sublime_uid and sublime_title are both used for tracking video usage.
Its a paid service that we are not using now. But who know, going forward, we might

That aside, the two new sublime_* columns also go into making the <a> and <video>
elements that we will need
Send back question video's UID along with preview information
Videos need just one action - play ...

Actually, even the need for this one action looks doubtful. It does nothing
more than get the videos UID. Thereafter, everything is done by SublimeVideos JS.

Just might remove the controller and associated view files itself
[First draft]: Integration of SublimeVideo into the site ...

Video lessons can now play on click. Video shown in a lightbox

Question videos still to be done