[bugfix] Store LastModified for domain perm subs + send as If-Modified-Since

[tsmethurst]

Description

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This pull request adds a LastModified column to domain permission subscriptions, which is used to store the Last-Modified header (if any) returned from a domain perm subscription fetch. This value is then transmitted as If-Modified-Since along with subsequent requests instead of the LastSuccessfulFetch value, as a more reliable way of ensuring that remotes can return 304 if nothing has changed.

(Also adds some docs I forgot to add before to the config.yaml example, oops.)

Checklist

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