Changing labels in editor switcher

Per https://trello.com/c/3PGGXjb8/14-3-switch-ve-wikitext-icon, we
want to match the labels that are currently in use on desktop: 'Edit'
(for Visual Editor) and 'Edit source' (for source editor).

Also making the buttons left aligned, since otherwise they look
pretty weird now.

Change-Id: I439279e3ac956f90812ae12a712f71f6f43cd28d