Fixes #37481 - Update cached value for manifest expiration date during import

[jeremylenz]

Our stored manifest expiration date relies on a Rails cached value which is held for 1 minute:

def manifest_expiration_date(cached: true)
          Rails.cache.fetch("#{self.label}_manifest_expiration_date", expires_in: 1.minute, force: !cached) do

This means that after a manifest import / refresh, the manifest expiration date may be inaccurate until the cache expiration time (1 minute). Because of this you must refresh the browser page to see the new manifest expiration date.

What are the changes introduced in this pull request?

1. During manifest import (and as a consequence, during manifest refresh), I found that calling manifest_expiration_date(cached: false) is enough to prevent stale data from making it to the user's screen.
2. If the manifest expiration date is not populated for some reason, don't display anything.

Considerations taken when implementing this change?

What are the testing steps for this pull request?

Delete and reimport a manifest. Visit the Manage Manifest modal.

Before:
You'll see a manifest expiration date of the Unix epoch (31 Dec 1969 or 01 Jan 1970)

After:
You'll see the correct manifest expiration date

[jturel]

Nice work @jeremylenz - thanks for the PR! :)

[chris1984]

Nice work @jeremylenz - thanks for the PR! :)

We miss you man ❤️

[jeremylenz]

Updated to better handle importing of already-expired manifests.

[jturel]

Nice work @jeremylenz - thanks for the PR! :)

We miss you man ❤️

Likewise!

[chris1984]

Works great, thanks @jeremylenz