As part of https://github.com/discourse/discourse-solved/pull/342, some discrepancies in the old implementation resulted in solved topics linking to posts that do not exist (dangling custom field value). Causing us to see the following when there are no `answer_post`
```
NoMethodError (undefined method `user' for nil)
```
This PR prevents the error.
This commit autoloads plugin files, and also extracts features into their own modules.
- `plugin.rb` is smaller
- external plugins like discourse-automation and discourse-assign have their own entrypoints
- solved filters as well
Natalie Tay