Fixes #12961: Add a success / error message when saving changes

[RaphaelGauthier]

https://www.rudder-project.org/redmine/issues/12961

[Normation-Quality-Assistant]

This PR breaks qa-test

-- stdout -- 
Your branch is behind 'NRM/branches/rudder/4.3' by 2 commits, and can be fast-forwarded.
  (use "git pull" to update your local branch)
Updating 12a04cd..31e02fa
Fast-forward
[...]
-- stderr --
From github.com:RaphaelGauthier/rudder-plugins
 * [new branch]      bug_12961/add_a_success_error_message_when_saving_changes -> bug_12961/add_a_success_error_message_when_saving_changes_pr
From github.com:Normation/rudder-plugins
   12a04cd..31e02fa  branches/rudder/4.3 -> NRM/branches/rudder/4.3
   0efb02d..a2edb83  master              -> NRM/master
Switched to branch 'branches/rudder/4.3'
Switched to a new branch '4.3_test'
No config file found, using default configuration

You should run ./qa-test in your repository to make sure it works.
You can also run rudder-dev merge https://github.com/Normation/rudder-plugins/pull/67 --test to test with upmerging.
After this, you can remove the qa: Can't merge ta
-- Your faithful QA

[fanf]

OK, merging this PR