Add application limits to CI Variables settings

What does this MR do and why?

For #362227 (closed)

Users are no longer allowed to add CI variables if they have exceeded or are about to exceed the maximum application limit for their respective group or project. This applies to group and project CI variables; admin CI variables are not affected.

Note that the popover from the design was not implemented here, since disabled buttons don't fire hover/focus events that would trigger the popover to show up.

Screenshots or screen recordings

Note: Limit has been set to 5 or 10 for testing purposes (default is 200):

CI/CD Settings	Without Limits	With Limits (alert on top of table)	With Limits (disabled button + alert at the bottom)
Group
Project
Admin (No Changes)

When deleting/adding a variable

Screen_Recording_2022-11-25_at_10.13.09

How to set up and validate locally

1. Download the patch below and apply it to your branch. This will set the application limit to 5 instead of the default 200.

set-limit.patch

git am < set-limit.patch

2. Go to the CI Variables settings (Settings > CI/CD > Variables) for either a project or a group.
3. If your project/group has exceeded 5 variables, the Add Variable button should be disabled and the max limit alert should show up.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.