Add Tags view to see all tags

 * Move TagsController to Profile::TagsController, so a top level TagsController can exist
 * Add TagsController
 * Add ActsAsTaggableOn::Tag.public_tags scope
 * Add "Tags" to main navigation
 * Add TagsController#index to display all public tags (public gists' tags)
 * Add TagsController#show to display a public tags' gists
 * Delete profile SASS since its declarations are shared across controllers
 * Move shared CSS declarations into shared SASS file
 * Add MyGists::Public for getting public resources
   * Define MyGists::Public.tags to get public tags for a given page
 * TomDoc edits