Make thentos-the-lib polymorphic in Config

[fisx]

Issue #174 must be done before this.

Just like with ThentosError and DB, a library user needs to be able to extend the configuration.

some preliminary thoughts (copied from an old redmine ticket): currently,

• runAction takes a reader state that contains the full config (backend, frontend, proxy).
• backend uses proxy config.
• frontend uses frontend config (for email confirmation url, i think).

we want (i think):

• proxy config is part of backend config.
• runAction is parameterized over config type.
• runAction in frontend can only see frontend config, in backend only backend config.

[ChristianSi]

Actually the thentos-adhocracy backend currently needs the frontend config to extract the fronturl url for confirmation emails. Which is somewhat doubtful since the Thentos frontend is not even running in that setup, so the frontend config actually refers to the A3 frontend.

A clean solution might therefore be to that a separate entry ("a3-frontent-url" or something like that) which is exposed to the thentos-adhocracy backend for email generations.