FR: Use labels to manage checks and notifications

**Proposal:**
Allow users to add labels to their checks so that they can create notifications rules for the outputs of any checks.
**Current behavior:**
Right now users can add a tag to their check to differentiate which checks will go to A or B when filtering in their notification rule, but the use of labels feels intuitive for the UI.

**Desired behavior:**
The user assigns a label to the check.

The user can filter for the outputs from that collection of checks by specifying a check label.

**Use case:**
This is especially useful for users who have the following scenario:
100 checks that need to report to notification endpoint A and another 50 that need to report to endpoint B. They don't want to filter for every check_ID in their notification rule. They might also not want to go into every check individually and add a tag. Ideally users could select multiple checks simultaneously and add a label to them to send them to a different notification endpoint.