feature: implement mentions

[rasikachafekar]

Implement functionality to track mentions for a handle

[philipbeadle]

Merged these changes to develop, waiting for review to merge to master.

[celestialhanley]

when clicking on a users name, it takes you to that persons profile page and displays their posts. example: clicking on @lindsey would display all of her mews.

[lucksus]

During estimation: we think it's fine to have mentions without read/unread notifications and see that as a different feature.

[science-girl]

As a user, I want to click on a handle within a mew and see the profile page of that user. A user handle that does not exist should (like twitter) not be clickable within a mew.