Added subscribers migration with accompanying model

[jbrooksuk]

Subscribers will simply register their email address and on an incident/component status change, they'll get an email.

[pedrommone]

Better extend with #36, no?

[jbrooksuk]

#36 is more related to third-party integration, I'd say?

[pedrommone]

Well, we can build a generic interface and make it easy to extend.

[manavo]

@jbrooksuk maybe add the soft deletes trait as well? (seeing as the DB column is there)

[jbrooksuk]

@pedrommone sure, but we'd need a place to catch subscribers, right?

@manavo good catch, added.

[pedrommone]

@jbrooksuk better make one step, then we figure out how to implement the #36