Add a form to post comments on images show page

* Add form to images show page
* Add nested route for comments
* Add comments#create controller action
* Add time since comment was posted to comment partial
Update nav heading and redirects

* Show links to "My Galleries" and "All Galleries" when signed in
* Redirect to galleries#show after signing in (instead of user#show)
Display a user's galleries as a <ul>
Remove redundant form label
Reorder nav links for more betterness