Arrayify some input to DocumentFilter::Filterer

Strings get passed through to the document filter from query parameters
like "world_locations=all". Arrayifying means we can be sure we're
dealing with an array and call #any? on it.
Fix some migration issues on markdown links.

- Add editorial remarks
- Replace whole markdown to prevent overeager
  replacements (e.g. replacing email addresses
  with mailto links in link text)
Fix document collection membership in speed tagger

Also eager load document collection groups to avoid an N+1 query
Only show edition problems if not state related.
Get rid of slightly confusing warning message.
Allow mailto and anchor links in markdown.
Copy change on link checker notices.
Fix incorrect regex syntax in link checking.
Remove accidental regex predicate.
Merge pull request #1043 from alphagov/basic-validation-for-markdown-links

Fixes to validation for markdown links

https://www.pivotaltracker.com/story/show/60397572
https://www.pivotaltracker.com/story/show/60397200
Merge pull request #1041 from alphagov/data-migration-for-markdown-links

Fix some migration issues on markdown links.

https://www.pivotaltracker.com/story/show/60397412
Use rack middleware to reject invalid UTF-8 bytes

This will prevent requests with strange characters in them raising
500 errors
Merge pull request #1042 from alphagov/fix-speed-tagger-document-collection-500-error

Fix document collection membership in speed tagger

https://www.pivotaltracker.com/story/show/59242784
Merge pull request #1038 from alphagov/59741752-utf-8-fix

Use rack middleware to reject invalid UTF-8 bytes

https://www.pivotaltracker.com/story/show/59741752
Merge pull request #1040 from alphagov/fix-announcements-atom-500-error

Arrayify some input to DocumentFilter::Filterer

https://www.pivotaltracker.com/story/show/59416132