add routes to pause/start/cancel transfers

This commit includes a paydown on technical debt incurred two years ago,
when I didn't know that it was bad to make custom Read and Show instances
for types. As the routes need Read and Show for Transfer, which includes a
Key, and deriving my own Read instance of key was not practical,
I had to finally clean that up.

So the compact Key read and show functions are now file2key and key2file,
and Read and Show are now derived instances.

Changed all code that used the old instances, compiler checked.
(There were a few places, particularly in Command.Unused, and the test
suite where the Show instance continue to be used for legitimate
comparisons; ie show key_x == show key_y (though really in a bloom filter))

Assistant/WebApp/routes

@@ -15,4 +15,8 @@
 /closealert/#AlertId CloseAlert GET
 /filebrowser FileBrowserR GET POST
 
+/transfer/pause/#Transfer PauseTransferR GET POST
+/transfer/start/#Transfer StartTransferR GET POST
+/transfer/cancel/#Transfer CancelTransferR GET POST
+
 /static StaticR Static getStatic