…ception classes

Otherwise we have to do string comparison on the exception names instead
of cleaner instanceof checks.

I was confused for a bit because the documentation says you can import
these classes but that wasn't working in practice when I first tried it.
Turns out we require 3.52.0 and the classes weren't exported until
3.53.0, just one release later!

Jest version 26 does not play well with ESM circular dependencies (to be
added in future commits) that work fine in production. Errors look like:

    RangeError: Maximum call stack size exceeded
    Exception in PromiseRejectCallback:
    internal/vm/module.js:321
            const module = await linker(identifier, this);

These errors go away with the next major version (27).

For actions on the group itself (e.g. membership management) rather than
the group's Source of datasets and narratives.

The Group management API (members, roles, overview, logo, etc) will live
under here.  Using a single namespace (…/settings/…) means we only add a
single new forbidden prefix for user-chosen dataset names in the group
(i.e. in addition to the existing …/narratives/… namespace).

Routes under this namespace are only visible to users with Read access
to the Group itself (not the GroupSource), i.e. access is limited to
group members even for public groups.

Previously, only GroupSource.group was available. Having the inverse,
Group.source, allows for more direct access.

However, maintaining both of these properties in the object constructors
results in a bad cyclic dependency. Since GroupSource.group is already
set in its constructor, adding Group.source as a computed property.

Update existing code to use this new property as a shortcut to create
new GroupSource instance.

This allows for reuse later with other endpoints.

DELETE: Extract the logic of creating multiple DELETE requests in
deleteResource to its own function.

GET: Extract the call to proxyResponseBodyFromUpstream in
sendSubresource to its own function.

PUT: Extract the logic of creating a PUT request in receiveSubresource
to its own function. This requires a new type, upstreamUrlExtractor, to
be used since the URL for these requests are dependent on the method and
headers, which are defined in the new function.

Read-write access for group owners, read-only access for all other
members of a group, and no access for non-members.