Add RESTful API endpoints for Groups customizations

[tsibley]

The goal is to allow for Groups using the new multi-tenant backend storage to self-manage logo and overview customizations previously managed via the special-cased files group-logo.png and group-overview.md.

Proposal

As a starting proposal, consider two new routes (exact paths up for discussion) × three methods (GET, PUT, DELETE) for six new endpoints:

/groups/:groupName/logo

    GET                           → 200 OK (Content-Type: image/png)
    PUT (Content-Type: image/png) → 204 No Content
    DELETE                        → 204 No Content

/groups/:groupName/overview

    GET                               → 200 OK (Content-Type: text/markdown)
    PUT (Content-Type: text/markdown) → 204 No Content
    DELETE                            → 204 No Content

These are just the "happy paths". 401/403/404 should also be returned as appropriate.

Implementation considerations

• If using the proposed routes, then they should come before more generic routes for datasets. Or, it might be preferable to change the routes instead to avoid conflict with user-provided paths, e.g. to /groups/:groupName/-/logo or similar. Settled on using the /groups/:groupName/settings namespace going forwards: Add RESTful API for managing Groups members #581 (comment)

• I don't think we need custom media types here but this could be discussed.

• Ok to accept just image/png for now. We can expand this in the future to more image/* types.

• The storage of this data can remain objects like group-overview.md, but we should ensure it can't conflict with other group data, like narratives. For example, /groups/…/narratives/group-overview shouldn't be able to touch the content uploaded for the group overview customization.

  This can likely be accomplished several ways. The best might be a layout change to the backend storage so that datasets and narratives live under a further prefix of the per-group prefix, e.g. s3://nextstrain-groups/blab/datasets/zika.json instead of s3://nextstrain-groups/blab/zika.json, so that the top-level is reserved for internal control data. I have a local prototype of this if we want to pursue that. Done in Prepare for adjusting the layout of the multi-tenant groups bucket #563 and Adjust layout of multi-tenant groups bucket #562.