Session storage for peerId (as discussed in #144)

[willstott101]

This basically moves the peerId into a query param of the connection rather than a Cookie. This means the client has control over how long it remains the same client.

I've moved the storage of the peerId to sessionStorage so that two browser tabs can connect and operate independently. The intention of then being able to share to two different people online, by suing two browser tabs.

Since the peerId isn't actually used for anything much currently (as mentioned in #110 )... the other option would be to make it completely ephemeral: see https://github.com/willstott101/snapdrop/commit/56ab2357a66797e56db1dfb489eda32150f8d3a5