Bug 21079: Unify metadata schema across backends

Bug 21079: Unify metadata schema across backends

This patch contains a database upgrade that takes appropriate metadata
properties relating to articles in the FreeForm backend and creates
metadata that corresponds with the metadata being created by the BLDSS
backend. This enables us to create templates that can display metadata
equally across any backends that contain this metadata;

To test:
- Ensure you have at least one article request created with the FreeForm backend
- Check the metadata for the request:
  => TEST: You should have properties such as 'article_title', 'article_author' populated
  => TEST: You should NOT have properties such as 'container_title' &
  'pages'
- Run the upgrade
  => TEST: For the same requests you should now have the following
  properties (if their values were originally populated):
    - container_title (this should correspond with what *was* 'title')
    - title (this should correspond with 'article_title')
    - pages (this should correspond with 'article_pages')
    - author (this should correspond with 'article_author')

Signed-off-by: Stephen Graham <s.graham4@herts.ac.uk>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>