Only offer VisualEditor if installed and enabled

We use the public mw.libs.ve.isAvailable flag to check this.
See https://github.com/wikimedia/mediawiki-extensions-VisualEditor/blob/b71e58/modules/ve-mw/init/targets/ve.init.mw.ViewPageTarget.init.js#L408

Bug: 63658
Change-Id: I3a6ccdc80e8bbc59f036e8ebf2c3970e3af9785e
Merge "Only offer VisualEditor if installed and enabled"