Only allow push owners to show, list or delete
Display form again when a rule can't be saved
Fix crash when a user has tags but no duplicates

The #uniq! method is not always chainable:

http://www.ruby-doc.org/core-2.1.0/Array.html#method-i-uniq-21

> Returns nil if no changes are made (that is, no duplicates are found).
Always sort and dedup tags, even when empty

Sorting and deduping an empty array works fine, and simplier code is
better than any marginal performance gains we're giving up here.
Use map instead of manually constructing array
Use local variable, not instance variable
Use nondestructive methods and chain everything
Count users articles directly, not through fetches