Allow user page markdown summary (BB-8759)

It would be very useful to be able to post a short introduction to yourself, your interests, etc on your project listing page. In particular this would allow posting something like a simple CV, or summary of your experience that may be hard to ascertain from just looking at your listed projects and contributions.

As for where to read the markdown from, I would suggest using the already-reserved <username>.bitbucket.org repository. As README.md already is used on the project page for that, I would suggest using USER.md within the top level of that repository.