luci-app-openvpn: add ovpn upload support & more

* add the ability to upload ovpn files directly,
  incl. appropriate uci entry in openvpn config
* add the ability to edit ovpn files directly ('file' mode),
  beside the 'basic' and 'advanced' modes for normal setups
* client side checks to validate instance name & template selection,
  incl. online error reporting
* automatically remove non-ascii characters & windows line endings
  from transfered ovpn file
* change from after_commit to after_apply hook
* remove misleading default values for Port & Protocol in Overview

Signed-off-by: Dirk Brenken <dev@brenken.org>