ejabberd: add option to enable/disable Message Archive Management, XEP-0313

Created by: JoKeyser

Hi! This addresses issues #703 and probably #740 (closed). I feel quite confident about the implementation of actions/ejabberd, I checked it running in the vagrant box with https://github.com/iNPUTmice/ComplianceTester (for how, see in #703). In contrast, I have my doubts whether the Plinth integration could be made nicer, I'm a bloody novice of django, and mainly copy & pasted things together. Please also let me know whether I can improve something else about this PR, to make review easy; it's my largest so far (and I thought this would be much easier :-)).

Here's a screenshot: