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
(cherry picked from commit 7f405134c710ca823782fdffc6f1e281d0183957)
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
(cherry picked from commit f7faf0777857b8c13e58b1297f4c5a0451c929c0)
Merge "Shorter, punchier text copy for the compact language links BetaFeature" into wmf/1.23wmf17