luci-proto-wireguard: handle multiple peers in imported configuration

When importing a fully configuration, import all peer entries from it
instead of non-deterministically merging all peer keys into one.

When importing a remote configuration as peer, only use the setting from
the peer section matching our local interface pubkey.

Also relabel the `Import peer configuration` button to
`Import configuration as peer` in order to be more explicit.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>