Show statistics of "how many instances added since X date" somewhere

[redshiftzero]

Something that has come up a few times when writing grants for FPF or SecureDrop is: "how many instances have been added since X date?"

Fortunately, it looks like on the new website we are storing the date a SecureDrop instance was added to the directory on the DirectoryEntry model in added (this model is in directory/models/entry.py).

Going forward, it would be very rad to have a way of displaying how many instances have been added in the last e.g. year (time period(s) TBD) in the Wagtail admin. An alternative suggestion would be to simply provide a CSV export from the Wagtail admin. This export should contain at minimum the landing_page_url, onion_address, and added, such that people writing grants may use this information with e.g. a spreadsheet program to compute the statistic of interest to them for their grant.

[harrislapiroff]

It's worth a note that the added field is specifically when the entry was added to the database and currently isn't admin-editable. Depending on how we want to measure that value (i.e., if we want to record the historical date when an instance was set up rather than when it was added to our database) we'll need to either make that field editable or add a separate field.