This was causing issues on some sites, having the const, because this really is heavily
dependent on upload speed. We request 5-10 URLs at a time with this endpoint; for
a 1.5GB upload with 5mb parts this could mean 60 requests to the server to get all
the part URLs. If the user's upload speed is super fast they may request all 60
batches in a minute, if it is slow they may request 5 batches in a minute.
The other external upload endpoints are not hit as often, so they can stay as constant
values for now. This commit also increases the default to 20 requests/minute.
This commit refactors the direct external upload routes (get presigned
put, complete external, create/abort/complete multipart) into a
helper which is then included in both BackupController and the
UploadController. This is done so UploadController doesn't need
strange backup logic added to it, and so each controller implementing
this helper can do their own validation/error handling nicely.
This is a follow up to e4350bb966
