Blocks deletion of context fields that are in use and updates the
"active usage" count to exclude use in archived flags.
- Before allowing you to delete a context field, checks if it is in use
by any strategies. If so, returns a 409 error.
- Updates what we count as "in use" to exclude flags that have been
archived.
BREAKING CHANGE: Context fields can no longer be deleted if they are in
use by active (non-archived) flags.
