FEATURE: include tag sidebars, migrate to objects setting

[awesomerobot]

This incorporates the tag sidebar functionality from: https://github.com/awesomerobot/discourse-tag-sidebars/ so that theme component can be deprecated.

Now this component will be able to show topic-populated sidebars for categories and tags.

This also:

• migrates to the objects setting type, which allows for easier category and tag selection using the respective setting types
• allows a single setting value to assign a topic to multiple categories and tags
• adds an acceptance test for tag sidebars
• adds a migration test
• renames the component to "Discourse Topic List Sidebars"

When a tag is assigned, it will take precedence over a category. If the same category or tag is used in multiple settings, the most recently created setting will apply.

Because this migrates a category slug to a category using getCategoryIdByName there may be some edge cases when a category slug doesn't align with the expected category name.