Prevent StreamingPayment uninstallation if pending streams remain

[area]

The main feature here is to prevent uninstallation of the extension in the event that there are some streaming payments unclaimed. It also:

• Reworks some calculations to prevent over/underflows. This is somewhat opinionated in that the intention is to 'cap' the calculations
• Allows editing of the interval, which is a requirement. While in an ideal world, editing the interval shouldn't be necessary as you could edit the amount, rounding errors meant that this should be editable as well. This is possible as we have moved to single-stream payments.

[area]

You mention that changing the interval is necessary to avoid rounding errors, but realistically what are the errors we're talking about? A few wei? I would be in favor of going the opposite direction and hard-coding the interval as 24 hours. We can provide a UI which converts different intervals (hour, week month, etc) if needed.

If tokens don't have, or have set decimals() to 0, then 'A few wei' could easily have significant value.

In terms of a hard-coded interval, even ignore editing existing streams: if we had it at 24 hours, we couldn't e.g. pay out 1 token that had decimals() as 0 every 16 hours.