Allow to add images to proposals and comments

[nidico]

Requirements:

• It should be possible to add one or multiple images to a proposal/comment
• Images should be automatically scaled to defined sizes.
• This should be allowed to original proposal author and by other users if the wiki attribute is set.
• The feature should be activateable on a per-instance-base.

Implementation plan:

• Use Kotti to store and scale the actual image (see #315)
• The Pyramid/Kotti based REST API publicly exposes the ability to view images in different predefined sizes.
• It also exposes an API to add new images
• The Adhocracy pylons process acts as a client against the custom REST API.
• Permission checks are done on Pylons side; Pyramid trusts the Adhocracy (Pylons) client
• The Pyramid configuration defines which sizes (thumbnail / logo / normal / whatever) are translated to concrete image widhts / heights, Adhocracy/Pylons isn't aware of that.

[xi]

We do not plan to use kotti for all instances anymore. So this will probably be implemented using lib.logo instead.