Move move cursor to the beginning of textarea (#425)
Move move cursor to the beginning of textarea (#425)

When modifying translation textarea programatically, move mouse cursor
to the beginning, because this is where users start reading from and
making changes.
participedia/pontoon,jotes/pontoon,mozilla/pontoon,mathjazz/pontoon,participedia/pontoon,jotes/pontoon,mathjazz/pontoon,mozilla/pontoon,jotes/pontoon,mastizada/pontoon,mastizada/pontoon,mozilla/pontoon,jotes/pontoon,participedia/pontoon,mastizada/pontoon,mastizada/pontoon,mozilla/pontoon,participedia/pontoon,mathjazz/pontoon,mathjazz/pontoon,mozilla/pontoon,mathjazz/pontoon