Plugin system rewrite

[rafalp]

This is a rewrite of plugin system inspired largely by my experience from writing a new plugin system for abandoned v4 effort.

Replace plugin list by plugin discovery in plugins directory

Instead of plugins.txt file that users will have to maintain, plugins could be discovered and loaded from dedicated plugins directory. This will simplify installation by users and logic for installing their dependencies on container's build time.

Implement proper action and filter hook types for plugins to use

Misago should implement custom action and filter hooks:

• action hooks let plugins run extra code when an event happens in code.
• filter hooks let plugins run extra code around the standard logic implemented by Misago, and in rare cases, replace this logic altogether.

Reason for using custom action hooks instead of Django signals is to provide proper safety net in form of types.

Write developer docs

Misago should have a developer documentation for new plugin system written as quickly as possible. The guide will be human-written, but the hooks reference will be generated from code, due to amount of work needed to maintain and otherwise keep it up to date.

Roadmap

• Replace plugin list with plugin directory
• Install requirements
  • Install .[dev] requirements by plugins/plugin-dir/pyproject.toml file
  • Install requirements by plugins/plugin-dir/requirements.txt file
  • Install requirements by plugins/plugin-dir/requirements-dev.txt file
• Include plugins
  • Include plugins in Python path
  • Include plugins in INSTALLED_APPS
  • Include plugins urls in urlpatterns
• Display plugins list in admin
  • Gather manifests from plugin/misago_plugin.py files
• Install plugins from pip using pip-install.txt file
  • Include plugins in INSTALLED_APPS
  • Display plugins on plugins list in admin
• Add plugin_data json field to some models, set GIN index on it
  • Implement PluginDataModel abstract model
• Implement hooks
  • Filter hook type
  • Action hook type
  • Template plugin outlet tag
  • Template outlet has plugins conditional tag
• Docs
  • Plugin structure
  • Plugin installation (local)
  • Plugin installation (pip)
  • Plugin tutorial
  • Plugin manifest
  • Plugin hooks
  • Plugin data
  • Template outlets
  • Template overrides
  • Plugin urls
  • List of links to other extension points in Misago (link to custom notifications)