Put new compact language links Beta Feature behind a feature flag

Add new $wgULSCompactLinks feature flag, defaulting to false (as is standard
for Beta Features). Only allow enabling of the feature if ULS position is
'interlanguage'.

Change-Id: Ia268c3a49b5aa14b6a00e33c7f01a61eba48e776
Shorter, punchier text copy for the compact language links BetaFeature

The title should be a noun, not a verb, and should use as simple language as
possible. The description should sell why it's better, rather than describe
what it does.

Change-Id: I72517ae8c48dfa7c5f294fea59f406d7b3cad96a
Merge "Put new compact language links Beta Feature behind a feature flag"
Merge "Shorter, punchier text copy for the compact language links BetaFeature"