Extension management

[stijn-uva]

Adds a worker that can install and uninstall extensions and webtool controls to manipulate it

Todo:

• Enable and disable extensions - requires changes to ModuleLoader which currently cannot read from the configuration settings database
• Better frontend UX - currently requires page refresh to reflect whether extensions have been uninstalled, remains possible to interact with extensions after initiating uninstall
• Configuration management - what happens to extension-related settings when they are installed/uninstalled? Should recurring jobs be deleted?
• Docker things - does the frontend also need the extensions installed if running in a separate container?

[dale-wahl]

Weird bug: One cannot update the admin tag to allow managing extensions. It looks like those admin settings are created when creating the database itself. We could add it to a migrate script, but perhaps there is another way.