Split exportRemotes out from syncDataRemotes; the parts of the assistant
that upload keys and drop keys from remotes don't apply to exports,
because those operations are not supported.
Some parts of the assistant and webapp do operate on both
syncDataRemotes and exportRemotes. Particularly when downloading from
either of them. Added a downloadRemotes that combines both.
With this, the assistant should download from exports, but it won't yet
upload changes to them.
This commit was sponsored by Fernando Jimenez on Patreon.
Note that get --from foo --failed will get things that a previous get --from bar
tried and failed to get, etc. I considered making --failed only retry
transfers from the same remote, but it was easier, and seems more useful,
to not have the same remote requirement.
Noisy due to some refactoring into Types/
