Sometimes a task is queued and scheduled for later (or for a retry too),
but the alert group is resolved by then. Skip notification in that case.

# What this PR does
If an OnCall template needs to contain a date relative to a date in the
alert response, currently there is no way for the template to add or
subtract time from a parsed date.

This PR adds a function that allows a time-window (e.g., 1s, 5m, 6h, 7d,
2w) to be converted into a Python timedelta, which can then be added or
subtracted from a datetime. An example usage might be:

```
{% set delta = alert.timeWindow | timedeltaparse %}
{% set time = alert.startsAt | iso8601_to_time - delta | datetimeformat('%s') %}
```

## Checklist

- [X] Unit, integration, and e2e (if applicable) tests updated
- [X] Documentation added (or `pr:no public docs` PR label added if not
required)
- [X] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will show up in the
autogenerated release notes.

Bumps [django](https://github.com/django/django) from 4.2.15 to 4.2.16.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/django/django/commit/6f9fea33137fee6416ff43b775aa9567440a23d3"><code>6f9fea3</code></a>
[4.2.x] Bumped version for 4.2.16 release.</li>
<li><a
href="https://github.com/django/django/commit/bf4888d317ba4506d091eeac6e8b4f1fcc731199"><code>bf4888d</code></a>
[4.2.x] Fixed CVE-2024-45231 -- Avoided server error on password reset
when e...</li>
<li><a
href="https://github.com/django/django/commit/d147a8ebbdf28c17cafbbe2884f0bc57e2bf82e2"><code>d147a8e</code></a>
[4.2.x] Fixed CVE-2024-45230 -- Mitigated potential DoS in urlize and
urlizet...</li>
<li><a
href="https://github.com/django/django/commit/705066d186ce880bf64142e47084f3d8df3c2352"><code>705066d</code></a>
[4.2.x] Fixed grammatical error in stub release notes for upcoming
security r...</li>
<li><a
href="https://github.com/django/django/commit/b07d4f2dea2e5884d3d13c981fd3f22dc8f2e915"><code>b07d4f2</code></a>
[4.2.x] Added stub release notes and release date for 4.2.16.</li>
<li><a
href="https://github.com/django/django/commit/e0579ce27746b04a37cf43559df445068fd2a781"><code>e0579ce</code></a>
[4.2.x] Added CVE-2024-41989, CVE-2024-41990, CVE-2024-41991, and
CVE-2024-42...</li>
<li><a
href="https://github.com/django/django/commit/ae0ca8345dd4a2469ac45211522f8d4bf5bc610c"><code>ae0ca83</code></a>
[4.2.x] Post-release version bump.</li>
<li>See full diff in <a
href="https://github.com/django/django/compare/4.2.15...4.2.16">compare
view</a></li>
</ul>
</details>
<br />


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