Add toast when variable actions/mutations succeed

What does this MR do and why?

For #392948 (closed)

This MR adds a toast that shows when a variable was successfully added/updated/deleted.

Why?: This makes it easier to know whether actions taken on variables succeeded. Also, the variables list will no longer show all variables at once with the ci_variables_pages feature flag enabled, so it will become even less clear whether actions succeeded unless we add this feedback.

Screenshots or screen recordings

page	screen recording
project settings	Screen_Recording_2023-03-28_at_17.25.45
group settings	Screen_Recording_2023-03-28_at_17.26.57
admin area	Screen_Recording_2023-03-28_at_17.28.37

Note

The way this is currently written, the updated toast is still shown even if the variable wasn't changed in any way:

Screen_Recording_2023-03-28_at_17.31.50

This behavior could be changed - I'm not sure if it feels confusing or reassuring, and I don't know of any examples like this to check in the existing codebase 🤔

How to set up and validate locally

1. git checkout 392948-add-toast-on-variable-change
2. navigate to a project
3. in the sidebar select Settings => CI/CD
4. expand the Variables settings section
5. add a variable - a toast should be shown
6. update a variable - a toast should be shown
7. delete a variable - a toast should be shown
8. navigate to a group, then repeat steps 3.-7.
9. navigate to the admin area, then repeat steps 3.-7.

To run tests

yarn install && yarn jest variable

MR acceptance checklist

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