Trim whitespace from titles on create or update

Some of the lists on the site are ordered by edition title. These don't
display correctly if the first letter of the title is whitespace. This
should remove any extra whitespace from edition titles.
Add data migration to trim edition titles

Remove extra whitespace from the start to help with sorting by title.
Merge pull request #399 from alphagov/trim-titles

Trim whitespace from titles on create or update