Update VE core submodule to master (8f8d29a)

New changes:
84a2e6f Reorganize initialization of default size in MediaSizeWidget
e8d0e8c Update OOjs UI to v0.1.0-pre (99068355d6)
f0e9ad9 Allow for the disabling of the MediaSizeWidget
25c4fce Tighten up context menu items

Change-Id: Id6c97cc3713d22490c7afd806eb9eee282fefd85