added delegatable to mediafile relations; new entity MediaFile to store files  with the adhocracy_kotti.mediacenter web service
added mediafile migration script
mediafile: added backref mappings
edit and show proposal mediafile
added options to disable assigning mediafiles to proposals

Conflicts:
	etc/adhocracy.ini.in
mediafile model: more DRY
mediafile: test __repr__
model: mediafiles to comments assignment.
ui to add images comments (javascript only)
Add missed file lib/helpers/adhocracy_service.py
Port "add mediafile" migration from sqlalchemy-migrate to alembic
Accept image/* mediafiles, not only image/jpeg
Allow to split mediacenter and staticpages services

This allows to use separate mediacenter and staticpages API endpoints by
defining the more special configuration options:

* adhocracy_service.mediacenter.rest_api_address
* adhocracy_service.mediacenter.rest_api_token
* adhocracy_service.staticpages.rest_api_address
* adhocracy_service.staticpages.rest_api_token

Conflicts:
	src/adhocracy/lib/helpers/adhocracy_service.py
Optionally allow unverified staticpages / mediacenter SSL API calls

Conflicts:
	src/adhocracy/lib/helpers/adhocracy_service.py
fixup mediacenter alembic down_revision
Merge branch 'feature-mediacenter-2.2.1' into deploy-ypart-2.2.1

Conflicts:
	etc/adhocracy.ini.in
	src/adhocracy/controllers/proposal.py
	src/adhocracy/lib/helpers/adhocracy_service.py