Allow providing edit summary for translations through tux

A text input field (shortcut key: "b"; same as EditPage) has been added below
the translation area so that an edit summary can be provided by the user.
It is disabled by default but gets enabled when a textchange event is emitted
in the translation box. This box appears only for users who can translate.
Pressing "discard changes" button or removing all the text from translation area
will also disable this box (same behavior as save button).

Bug: T48924
Change-Id: I717ad9b6a615a8b31e9a47042273a30288ba7910