Install and update plugins from remote locations

This is an Nlnet milestone.

“Develop ability to install and update plugins from remote locations”

Background:

Enhance self-hosting of instances

Building upon our previous work that added the possibility to develop plugins, which is of particular interest to self-hosted instances, we want to continue work on that to improve the experience, now we have some groups writing their own plugins. Based on user feedback we understand the need for more documentation and examples, an easier way to package and install plugins, and ability to publish/install plugins in a way that updates can be handled easily.