Consider moving Lob integration code out of the "loc" package

[toolness]

The Lob code was originally part of the "loc" package because the only feature using Lob was the letter of complaint.

However, now EvictionFreeNY and NoRent use it, and I think even the HP Action admin has support for staff to serve landlords through it. It feels weird that such functionality has to import from loc so we might want to move the Lob-related code to a separate package.

Honestly though, we might also want to consider just having a separate SentLobLetter model that stores all the info. This would make it easier to implement things like #2033 and would avoid the need for funky mixins like the ones introduced in #2037. However, it would also involve a pretty major database migration, which might be stressful.

Still, at the very least, though, it might be a good idea to move the common Lob code out of "loc"!