Introduce experimental features flag

Introduce wgContentTranslationExperimentalFeatures
Setting it true enables a number of experimental features that
are either not planned for next release or not yet ready to face users.

Default value is false

Merged wgContentTranslationWYSIWYGEditor to this configuration

Change-Id: Ide9dca5edae90db3057f037e2ec29dca0c596a41