organisation: allow many themes for each working group

Move the current single foreign key theme to a manytomanyfield
themes and migrate the data as well.

Since it is bad practice to mix schema and data migrations this
calls for three separate migrations:

1. Add the many to many field (themes)
2. Migrate the data (add value of theme to themes)
3. Remove the foreign key (theme)
organisation: update templates and tests for many themes in working groups

Since themes are now in a many to many relationship with working
groups that must be reflected in both templates and tests.
Merge pull request #152 from okfn/multiple-themes-in-wg

organisation: allow many themes for each working group