loopd: re-add GetLsatTokens method in gRPC

This is needed not to break existing client binaries, e.g. `loop listauth`,
Terminal Web, RTL.

The API should be removed in a couple of releases. For now, GetLsatTokens
just prints a warning message about the API being deprecated and that the
client binary should be updated, and calls GetL402Tokens API, as a wrapper.

Type LsatToken used by GetLsatTokens in the past was renamed to L402Token,
but this does not affect binary encoding, so type L402Token can be used
(as part of TokensResponse) without breaking backward compatibility.

Updated release_notes.md.

See lightninglabs#730 (comment)

loopd/perms/perms.go

@@ -91,6 +91,10 @@ var RequiredPermissions = map[string][]bakery.Op{
 		Entity: "auth",
 		Action: "read",
 	}},
+	"/looprpc.SwapClient/GetLsatTokens": {{
+		Entity: "auth",
+		Action: "read",
+	}},
 	"/looprpc.SwapClient/SuggestSwaps": {{
 		Entity: "suggestions",
 		Action: "read",