As a wiki administrator, I want to easily install WikibaseLexeme in a wiki without falling for any hidden traps.

Problem:
WikibaseLexeme depends on the UniversalLanguage selector and cldr extensions, but this is not obvious at all. If they are missing, it mostly appears to be working, you just can’t edit senses (see also T215500 for a description of the same problem).

Example:
https://wikidata-lexeme-lua.wmflabs.org was missing those extensions until I realized what the problem was (which took some debugging).

Screenshots/mockups:

BDD
GIVEN
AND
WHEN
AND
THEN
AND

Acceptance criteria:

• The dependencies are documented on https://www.mediawiki.org/wiki/Extension:WikibaseLexeme
• If they are missing, prominent warnings are displayed on-wiki, where editors that don’t read the mw.org documentation (such as yours truly – because 90% of the time it’s just git clone + wfLoadExtension() anyways, so why bother?) will see them

Open questions:

• extension.json can define dependencies on other extensions. Is there a reason why we aren’t using that for this? (Though I haven’t tested what kind of warnings this produces if the required extensions are missing.)