Storage of worksheets and exercises

[Nick Chadwick]

These should be migrated from the filesystem to the database, to facilitate
web-based content management, making them searchable, having extra columns
that store other properties of exercises.

[Matt Giuca (mgiuca)]

We will wait until this is done, then we can show the proper names of exercises in
the 'sidebar' (contents list of a worksheet), rather than their xml names.
Note that it isn't currently possible, because we don't open the XML files until
later in the worksheet generation process.

[William Grant (wgrant)]

Work ongoing in new-dispatch.

[William Grant (wgrant)]

Issue 71 has been merged into this issue.