At this moment, this feature is under a site setting named
lazy_load_categories.
In the future, categories will no longer be preloaded through site data.
This commit add information about categories in topic list and ensures
that data is used to display topic list items.
Parent categories are serialized too because they are necessary to
render {{category-link}}.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
This feature can be enabled by choosing a destination for the
`shared drafts category` site setting.
* Staff members can create shared drafts, choosing a destination
category for the topic when it is published.
* Shared Drafts can be viewed in their category, or above the
topic list for the destination category where it will end up.
* When the shared draft is ready, it can be published to the
appropriate category by clicking a button on the topic view.
* When published, Drafts change their timestamps to the current
time, and any edits to the original post are removed.
