Refactor basic auth

[maximbaz]

• Use a shared tabs.onUpdated instead of subscribing multiple times
• Extract onAuthRequired into a separate global function

@erayd I'll comment inline on a couple of particularly note-worthy points.

[maximbaz]

Thanks @erayd for the confirmations, any other feedback before this is merged?

[erayd]

@maximbaz No other feedback at this point - I like what you've done, go ahead and merge it :-).