Design: Investigate grouping options for the notification list

Problem

Currently, on the to-do list, notifications are not grouped in any way, and each event shows up as a separate row in the list. For instance, if there are multiple updates on a single issue or MR, those items may show up anywhere in the list, and will not be presented together.

This can create unnecessary noise and confusion for some users. This problem will increase as we start introducing more notification events to the list.

WIP proposal

In our initial vision designs and in the prototype, we've been exploring options for grouping notification events sensibly. But, we've yet to really dig into how should grouping of notification events work and look from a design POV. This issue will be for that work.