Tutorial: rst -> html conversion and custom global definitions

[Steven Bird]

The rst->html conversion process seems to use an old version of
'definitions.rst', perhaps stored in its document database. A consequence
of this is that '|NLTK-URL|' gets expanded to 'http://nltk.org/'. Is there
a way to customise these definitions for a given instance of IVLE?

[William Grant (wgrant)]

As discussed today, we should implement a general include virtual file system for the
worksheets. definitions.rst would then become just another file, and would live in
the database.

We could also use this VFS to serve media for the worksheets.