Improve report notifications in Slack

[dnlstpnv]

1 . Add the ability to tag users by ID in Slack notification settings:

• An additional field where you can input user id using format <@id> <@id> ...
• Checkbox Notify user if run failed to ensure that user tagging only works in case of failure.

2. Do not show :x: FAILED 0 if run has no failures:

• Checkbox Always show failed count In case of true, always display the number of failures; if false, show only if failCount > 0.

[TetianaKhomenko]

related to https://github.com/testomatio/app/issues/924

[ethan-bm]

My company is interested in an option to "only show failed or passed tests" in the webhook report, even if the suite has tests that are skipped.