S3: Default to signature=v4 when using an AWS endpoint

S3: Default to signature=v4 when using an AWS endpoint

* S3: Default to signature=v4 when using an AWS endpoint, since some
  AWS regions need v4 and all support it. When host= is used to specify
  a different S3 host, the default remains signature=v2.
* webapp: Support setting up S3 buckets in regions that need v4
  signatures.

For the webapp, went ahead and added all current S3 regions
(except govcloud, which  is not usable by everyone).

Sponsored-by: Dartmouth College's DANDI project