gluon-config-mode: rename configmode to config-mode (except in lua variables where it is still called configmode)

It is now possible to specific multiple branches like this:

config branch stable
  option url ...
  list pubkey ...
  ...

config branch testing
  ...

and select one in autoupdater.settings:

config autoupdater settings
  option branch "stable"

gluon-config-mode: do not depend on luci-mod-failsafe (gluon-luci-admin will provide it's functionality)