Extend last-modified calculations for FRUS TEI to the tags collection

[joewiz]

In #396 we implemented handling for Last-Modified and If-Modified-Since for TEI resources. For FRUS TEI, we derive the last modified date from the last modified dates of the TEI file itself and the associated bibliography file. However, volumes' landing pages are also constructed from the data in the https://github.com/HistoryAtState/tags repository. As a result, updates to a volume's tags such as HistoryAtState/tags@850d8b3 can be hidden from view, since eXist and Nginx think that the volume hasn't been updated. The last-modified calculation should extend to the data in the tags collection.

[joewiz]

As reported by @joshbotts, this also affects updates of the POCOM data shown in SecState bios like https://history.state.gov/departmenthistory/people/albright-madeleine-korbel.