This adds a 5 minute auth cache to speed up the process of deleting
old devices. It has the following nastinesses (mainly due to being
written on a flight whilst juggling kids):
* the auth cache is done as context attached to MatrixChat.
one could argue that it should be per-client instead, but we don't
yet have multiple clients.
* the auth cache is only maintained currently in DevicesPanelEntry
(i.e. set & invalidated). One could argue that it might be better
maintained in InteractiveAuth.js or a dedicated cache object
abstraction, but given the only use I can think of is when managing
devices, perhaps this is good enough for now.
