Descriptions can currently only be accessed directly via the Entity data structure, as obtained via [[ https://www.mediawiki.org/wiki/Extension:Wikibase_Client/Lua#mw.wikibase.getEntity | `mw.wikibase.getEntity` ]]. This means they are tracked using the `X` or `O` aspect for now; given we will track which part of the Entity data structure is actually used (T90436), we we should track description usage separately, using a new `D` aspect.
Steps to get this rolled out:
[x] Create the usage aspect
[x] Add `'D'` to `$wgWBClientSettings['disabledUsageAspects']` (T177153)
[x] Make `mw.wikibase.description` and `mw.wikibase.getDescriptionWithLang` add a description usage along with the other usage (T177154)
[x] Deploy description usage tracking for Lua (513f79bf94a560d3baa736e12d06eba55165ff1a)
[x] Remove `'D'` from `$wgWBClientSettings['disabledUsageAspects']` test-wise on a few wikis (all small wikis, `elwiki`, …) (T177155)
[x] Evaluate data from T177155
[] Remove `'D'` from `$wgWBClientSettings['disabledUsageAspects']` on a few more wikis (T178515)
[] Remove `'D'` from `$wgWBClientSettings['disabledUsageAspects']` on commons (this might be the wiki where this has the biggest effect)
[] Remove `'D'` from `$wgWBClientSettings['disabledUsageAspects']` on all wikis
[] Use compact representation of diffs in EntityChange. (T113468)
[] Make `AffectedPagesFinder` take description usage into account (T176417)
[] No longer save other usage along