Fall back to the first available editor when the preferred editor is not available
If a user used an editor on one platform so it was stored as their
preference, then used Flow on another platform where that editor
is not supported, we'd try to switch to their preferred editor,
fail, and put the editor in an unusable state.
This could happen in many circumstances, but was especially
common when using the mobile site: VisualEditor marks itself
as unsupported when using the minerva (mobile) skin.