Slack on-call status notifications are delayed longer than expected due to overzealous rate-limiting

[mastercactapus]

Describe the Bug:
When Slack on-call status notifications are triggered in quick succession, subsequent notifications can be significantly delayed (in this case, sent ~16 minutes after the triggering event). This behavior is unexpected and not clearly communicated, leading to confusion for users who receive updates well after changes occur.

Steps to Reproduce:

1. Configure a GoAlert schedule and enable Slack on-call notifications.
2. Trigger multiple on-call status changes/updates within a short period (e.g., adding and removing people from the on-call rotation in quick succession).
3. Observe that after a couple of notifications, subsequent alerts are delayed before reaching Slack.

Expected Behavior:
Users should receive Slack on-call status notifications promptly after the on-call state changes, or at most with a minimal delay. If rate-limiting is enforced, it should be clearly documented or communicated within the UI to inform users that notifications are pending/delayed.

Observed Behavior:
A Slack notification scheduled at 11:25 AM was not delivered until 11:41 AM. The delay was attributed to rate-limiting logic that, while helpful in preventing spam for mediums like SMS/voice, appears to be overly aggressive and not clearly indicated for Slack notifications.

Application Version:
All versions as of v0.33.0

Additional Context:

• Rate-limiting logic currently treats Slack notifications similarly to SMS/Voice, causing unintended delays for Slack updates.
• Users have no direct visibility into pending or delayed messages, resulting in confusion. An indication or banner on the schedule page for delayed notifications would help.
• Adjusting these rate limits or providing more transparency is desired to improve the user experience.