Add ability to follow a legislator

[rogerscg]

Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
Similar to the "Follow" button for a bill, a user should be able to follow a legislator, getting emails/notifications for proposed bills, co-sponsors, and votes.

Describe alternatives you've considered
N/A

Additional context
N/A

[jamesturk]

moving into the improved linkage milestone, depends on openstates/issues#22