The ETag caching system we use assumes that all notes are sent in a
single response, and makes that response the cached one. When a note
is subsequently updated, the ETag is invalidated and the note thrown
away.

This is incompatible with pagination, which is currently behind a flag
(appropriately, called `paginated_notes`. I *think* it also means that
the clients end up parsing the cached response every poll-interval,
which is less than ideal.

One approach is to remove the ETag from this endpoint altogether, but
this has some costs in terms of additional server load.

In this commit, I introduce a mechanism to make the ETag conditional,
and skip adding the ETag if there are any notes in the response. This
means that:

* Pagination will work correctly
* The browser will only ever cache the empty response

This means there will be one additional HTTP request before the ETag
is applied, but I think it's overall positive in its own right, and
allowing paginated notes to work is a big plus too.