Hacked in *very rough* support for creating/updating/deleting doc
units, as well as some sketchy validation tests. Need to work out
how to abstract this logic for all DescribedEntities but there are
difficulties due such entities all needing to implement methods for
adding removing descriptions. Ideally this should be done by manip-
ulating the entity bundle directly in a transparent manner (i.e.
without knowing anything about the model) but that will require more
thought and refactoring.
More sketchy work on multiple descriptions, this time concerning the
rendering of them on the view page.
Add a test for (incomplete) linking via annotation functionality
Add a test for uniqueness errors on identifier fields
Add another uniqueness test
Initial attempt to refactor the description creation code into a
general trait. This is complicated by the unfortunately convoluted
process of adding a new description to an existing entity, which we
should fix.
Use the same composed action method for update, since if there isn't
already a description with the given name, it will add it.
Revert "Use the same composed action method for update, since
it did not properly cater for filling the form with the right
id data to update the particular description properly, and ended
up adding one instead.

This reverts commit 1e954f59628dbef9027aa7d07c9eff42b361f7d5.
Fix flash message
Ensure item id is present in POST data
Remove unnecessary item fetch and fix formatting
Various tweaks to description template.
More logging on update
Add markdownj dependency
Minor style tweak
Log form errors whilst debugging templates.
Minor renaming
Fix in missed serialization values
Flesh out repository display, rendering text fields with markdown
Moved DatePeriod to DocumentaryUnitDescription model