Update VE core submodule to master (ba8872e)

Replace instances of target.$document, which was removed
in VE core.

New changes:
8083623 [BREAKING CHANGE] Remove $document cache from target
9256158 Cleanup target properties
cc0d71f Multi-surface fixes
ba8872e Localisation updates from https://translatewiki.net.

Change-Id: Ibff83cae770c056ed64bcf438ed74b44d929cdc0
Merge "Update VE core submodule to master (ba8872e)"