Generate sessionId and use it as cache buster for avatar images

Notes:

• The first time you load the website, avatars will be loaded twice. Once without the cache buster and once with cache buster. After that, if you navigate around, all avatar URLs will have a cache buster.
• The reason is the first time the page is loaded, the user is considered anonymous until we verify their login status. Only when they are logged-in do we generate a cache buster.

Closes #142 (closed)