Add `is_accepted` property to talks

This property will be useful for taking actions when a talk has been
accepted.
Display "accepted" badge for accepted talks

An "accepted" badge is being added to the user dashboard page. For users
who submitted multiple talks, this will serve as an easy way to know
which talk(s) was(were) accepted. It will hopefully remind them to make
sure its(their) description is up to date.
Merge pull request #44 from PyGotham/accepted-talks

Accepted talks