`discourse-common` was created in the past to share logic between the
'wizard' app and the main 'discourse' app. Since then, the wizard has
been consolidated into the main app, so the separation of
`discourse-common` is no longer useful.
This commit moves `discourse-common/(lib|utils)/*` into
`discourse/lib/*`, adds shims for the imports, and updates existing
uses in core.
Include categories when fetching admin/web_hooks and make
'extras' more useful. 'extras' is the mechanism we use to provide
context for rest objects.
However, previously:
* When you fetched many objects, extras was only set on the ResultSet,
not on each object,
* If you need derived data from extras, there wasn't a sensible place to
put this code. Now, you can create an 'ExtrasClass' static field on
your rest model and this class will be used for your extras data,
