fixes #19479, #10587, #19500 - two pane notifications are visible

- notification is now inside the content div, allowing two-pane server
responses to include it.
- refactored all notifications to use notification helpers (notice,
warning and error) instead of direct flash manipulation
- also added the usage of flash now via the helpers, and fixed
it in a couple of places it was used incorrectly (mostly clone rendering
which does not redirect at all).
- fixed consisteny to remove trailing dots from toast messages (as it
seems most strings are without dot suffix).
- safer handing if notification div or its data is undefined.