Reader: Only show Edit button in nav bar when there are tags to edit

Note: this only covers the initial load.
Reader: Add/Remove edit button when adding/removing first/last tag
Reader: Remove unnecessary header declarations
Reader: Rename TopicChanged and topicChangedBlock for clarity

See
https://github.com/wordpress-mobile/WordPress-iOS/pull/2436#discussion_r17709426
Reader: Hide/show edit button as appropriate on the Followed Sites page
Reader: Whitespace
Reader: Remove unneeded empty parameter list
Merge pull request #2436 from gcorne/issue/2365-hide-edit-no-tags

Reader: Hide edit button when user is not subscribed to any tags