Adds a beta feature for "Other project" sidebar

It introduce an optional dependance to BetaFeature extension

Adds two boolean configuration options:
otherProjectsLinksBeta to activate the beta feature
otherProjectsLinksByDefault to activate the other project sidebar by default

Relies on change I170e7253825c8dab5cad38e6b0ba59f28572efbf in MW core
"Expands" change I93767d3db76f694f2d37bbf2f2fecaf7643f4e3b in Wikimedia config

Change-Id: Ib4014253016db1c3d6b624be9ebbdaf452115145