Allow to specify a subset of firms per email

[stheid]

Currently the notifications of all firms are sent to all emails. it would make sense to be able to specify for each email, which firms it wants to be notified for.

This could be done eighter by building a real database with the tables users, firms and userToFirm or by simply maintaining the firms as a comma separated list, after each email.

adding the information to email.txt makes it easier to maintain, but harder to extend if the tool gets more features.

I think plaintext is king.

max-musterfrau@mail.org  <firm1>,<firm2>
dieder@t-online.de       <firm4>

maybe allowing the definition of shorthands:

wo=Wochenmarkt
ed1=Edeka musterstraße
ed2=Edeka waldstraße
max-musterfrau@mail.org   wo,ed1
dieder@t-online.de        ed2