Let controllers create canister bundle

[ninegua]

Problem

One application often has more than one canisters, so it makes sense to donate from a single allocation to a group of canisters when any one of them runs low on cycle balance.

Plan

1. Allow canister controllers to group a list of canisters together to create a bundle. A canister can only be put in one bundle.
2. Have a separate page that shows bundle specific info. This can also serve as as "donate to me" landing page.
3. Allow user to add bundles to their allocation list.
4. Existing per-canister allocation will be replaced by bundle if the canister is in a bundle.
5. Notify users when bundle member list is updated.