Add Favorites support to rovers photos

[mikelax]

In #3 initial support was added for interactive messages and the Favorite feature to apod.
As a user, I want to be able to favorite a rovers photos photo as well.

It should store similar attributes as for apod but with a couple of additional ones specific to rovers:

Attributes to store:
slack user id, Team, user, channel, timestamp, photo ID
Rovers specific - Sol, earth_date, Camera, Rover, img_src
We may just store all the rovers specific attributes in a single json object.