Add talk list and detail pages

Two new views are being added, `talks.index` and `talks.detail` to
display the talk list and detail view of a talk, respectively. Accepted
talks will be listed alphabetically.

The accepted talks can be accessed through a new property on `Event`
called `accepted_talks`.

In order to allow talks to be marked as accepted without them appearing
on the site ahead of time, a new column is being added to `Event` called
`talk_list_begins`. It will contain the date and time when the talk list
will first appear on the site. There is also an associated property
being added named `talks_are_published`. It is being used to control the
presence of the talk list in the menu, the accepted label in the user
dashboard, and whether or not the new views are accessible.
Merge pull request #47 from PyGotham/talk-list

Add talk list and detail pages