Refactor display of current/completed exercises

I couldn't work out a way to have the `gallery >>` links next to each submission that worked with any alternative layout I could come up with, so added an alternative route to get to them. When viewing one of your completed exercises, you are now provided with a link to "See someone else's solution to this exercise." 

There's quite a lot of duplication between the user and account pages - perhaps the account page could be limited to the gravatar, api key and email settings, and a link to the user page added to the user's menu under the heading of "profile" or similar?

* Use tables instead of `ul`'s for display of completed exercises.
* Add "View someone else's solution to this exercise" link to the completed exercise page (to replace gallery links).
* Create "Profile" presenter to clean up view code for account and user pages.

Closes #1151