refactor proposal acceptance test
refactor proposal acceptance test

This makes the proposal acceptance test more modular so that some parts
can be used in other tests (e.g. comments).

The changes include:

-   Expose a fixture "proposal" that creates a single proposal.
-   Remove class to force a clean database for every test (make no
    difference because there currently is only one test).
-   Move some functions to a separat "shared" module.

xs2maverick/adhocracy3.mercator,fhartwig/adhocracy3.mercator,liqd/adhocracy3.mercator,fhartwig/adhocracy3.mercator,liqd/adhocracy3.mercator,xs2maverick/adhocracy3.mercator,liqd/adhocracy3.mercator,fhartwig/adhocracy3.mercator,xs2maverick/adhocracy3.mercator,fhartwig/adhocracy3.mercator,xs2maverick/adhocracy3.mercator,xs2maverick/adhocracy3.mercator,fhartwig/adhocracy3.mercator,liqd/adhocracy3.mercator,xs2maverick/adhocracy3.mercator,liqd/adhocracy3.mercator,fhartwig/adhocracy3.mercator,liqd/adhocracy3.mercator,liqd/adhocracy3.mercator,fhartwig/adhocracy3.mercator