We add a new field delete_at in the channels template.
This field is then searched in the SearchChannels function.
Also added tests to verify that archived channels are searched
properly, and also indexed correctly.
https://mattermost.atlassian.net/browse/MM-62295
```release-note
- Now archived channels are searchable with ES/OS if TeamSettings.ExperimentalViewArchivedChannels is enabled.
- If there are old channels which were archived before a bulk index was run, users would need to purge indexes, and do bulk index again. Because those old archived channels are removed from the index when a bulk index is run.
```
Co-authored-by: Mattermost Build <build@mattermost.com>
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