WIP: added question_tags join table

The welcome#index is currently broken because of the introduction of the question_tags join table. We're trying to set up a many-to-many relationship between tags and questions. We want to call 'tags' on a question (question.tags) object to get an array of tags and display each one of those tags for a specific question. We also want to do the same thing with a tag object (tag.questions).