add normalizeChannel helper for consistent indexing/display across clients

[mmckegg]

I'm using this function in quite a few places, and it should really be consistent across clients, so I've added it to ssb-ref.

• enforce max length of 30
• lowercase
• no spaces

When merged, will be used in ssb-backlinks, patchwork, patchcore

Okay if I merge @clehner @dominictarr ?

[mmckegg]

Since no one has objected to this being merged, I'm going to go ahead and do it!