Add Tags view to see all tags

[simeonwillbanks]

• 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