Remove pluralization in AAIB Report type

This commit removes pluralization on report types.
Merge pull request #167 from alphagov/update-aaib-schema

Remove pluralization in AAIB Report type
Use more sensible redirects on NotFoundError

It's weird to redirect to `manuals_path` when rescuing from a
`NotFoundError` -- it's a hangover from when we didn't have any
permission handling.
Allow User to select multiple aircraft categories

- adding select2 gem for doing nice multi selects
- removing partialised extra form fields that were left in
Merge pull request #168 from alphagov/use-more-sensible-redirects

Use more sensible redirects on NotFoundError
Merge pull request #166 from alphagov/allow-multiple-tags

Allow User to select multiple aircraft categories
Importer set up initially to import AAIB reports

From `import/metadata` (json files, see spec/fixtures/import) and
`import/downloads`, import AAIB reports.

Also create an optional decorator used to validate that all attachments
for a document have been referenced in the body of that document.

Worked on by myself and @bestie
Handle duplicate titles.

 * For documents which already have a newer version, just skip it.
 * For documents which have an older version, delete that version and
   import the newer instead.