Deleting a custom field will break saved filters

Issue Summary

Currently, deleting a custom field will break any saved filters that reference the field. It would be nice to give some warning of this.

Steps to Reproduce

1. Create a saved filter that references an existing custom field by name e.g

   "Custom Field to delete" = A

   Check to see that this filter returns issues successfully:
   

1. Delete the custom field referenced in the saved filter
2. Go back to the filter in step 1 and observe that it is now broken with the error "Field 'Custom Field to Delete[Checkboxes ' does not exist or you do not have permission to view it":
   

Expected Results

Any breaking changes resulting from deleting a custom field should be flagged in the UI.

Actual Results

There is no indication that deleting a custom field could result in broken filters.

Workaround

You can restore a deleted custom field from Trash for up to 60 days after its deletion.