Revert "Revert "Preserve textarea state when building WikiEditor UI""

Ib97f47ef1d66420682bd429c9c12e66c3392e77d Didn't cause
problems so this is now safe to reapply (to wmf2).

This reverts commit 908e7cc56c4ef5db2ab0d213b2dbd8c8034a0da7.

Change-Id: I510536959b58ff0adacd7908f6150c95bfdb3ec1
(cherry picked from commit b5696dc435727bde5f6ae55d616e845519a3a174)