luci-base: Improve change application message

luci-base: Improve change application message

As a native English speaker the luci message displayed when clicking
'Save & Apply' has a somewhat uncomfortable English phrasing of "Waiting
for configuration to get applied".  It could be improved by replacing
'get' with 'be' but once you've got that far you might as well replace
this with the shorter & simpler "Applying configuration changes".  This
is also technically more correct since luci/openwrt should only be
updating/restarting processes that are related to the changed areas, not
changing the entire configuration.

As a result of that, change both the 'success' and 'rolled back'
messages to "Configuration changes have been applied" & "Configuration
changes have been rolled back"

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>