As a wiki administrator, I want to be able to install Wikibase on an existing wiki without disturbing its current users.

Problem:
Wikibase unconditionally overrides MediaWiki’s standard search box with a custom search box that only searches items. That’s fine for Wikidata, and for other wikis where the Wikibase repository is the main functionality, but it’s a problem if there’s also other interesting stuff on the wiki, as on the OpenStreetMap wiki where @Yurik recently set up Wikibase.

Acceptance criteria:

• There is an option to disable the custom search box and leave the standard search box in place.

Workaround
This is a super ugly hack, but it works, at least on my local wiki:

// add this after the Wikibase extension is loaded
$wgResourceModules['wikibase.ui.entitysearch']['scripts'] = [];

However, this may not be possible in more complicated setups, depending on how settings are loaded there.

Implementation note:
The search box is implemented in the wikibase.ui.entitysearch ResourceLoader module, which is added to output pages in the RepoHooks::onBeforePageDisplay function. Presumably it shouldn’t be too difficult to make that hook handler check for a config variable.

It should be noted alongside the config var that it is intended to be removed once T190454 is resolved.