Set extAdmEnabled=true when enabling frontend editing for extension compatibility
When frontend editing is enabled in t3lib_tsfebeuserauth we create a new instance of the frontend editing controller which serves as an indicated that frontend editing is active. In the past, a member variable named extAdmEnabled was the flag that indicated frontend editing was active.
Extensions may rely on $BE_USER->extAdmEnabled so it should continue to be set when frontend editing is active.
(issue imported from #M10599)