Plugin system update

Misago implements a plugin system, but while working on Misago v4 I've developed new one that's superior to current one. I would like to bring those improvements back to current Misago version:

• Plugins are kept within plugin directory.
• Each plugin has top-level directory that contains its requirements files (either pyproject.toml or requirements.txt are supported), its Python package with misago_plugin.py manifest file and, in future, client package that contains plugins for use in JavaScript application.
• Plugins have misago_plugin.py manifest file which enables plugin authors to provide information about plugin that can be used by Misago on list of installed plugins in admin panel.
• Plugins use action and filter extension hooks to extend or replace standard logic implemented by Misago.
• Some of Misago's standard models will have a plugin_data JSON field to let plugins store small bits of in Misago's tables for quick access.

Plugin directory really simplifies container bootstrap and keeps all plugins in same place.

Because individual plugins directories are not Python packages but directories that contain Python packages, those plugin dirs can be git clone'd from github as an installation method.

Each plugin's requirements are taken care of separately. Unwieldy and nightmare to maintain requirements-plugins files are gone.

Roadmap item: #1524

New plugin discovery logic has been merged. Misago no longer relies on plugins.txt file to find plugins. Instead there's dedicated plugins directory that is searched for plugins on app's start. Plugin installation and discovery is mentioned in new plugin guide.

Biggest change from current system is that plugins may be added to Misago directly through git clone, and there's no need to edit any files to activate them.

Next to do: List of installed plugins in admin panel.

First iteration of plugins list in admin panel:

I am not happy with it and I'll try to improve it before merging. 😀

Final version:

First plugin in admin dashboard: