Request: hold all of next month in Store?

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature
**Is your feature request related to a problem? Please describe.**

In the Agenda view, one of the headers is "next month." Based on that working, I expect it should contain all the events that fall in the next month.
However, the model only holds 6 weeks at a time by default. This means the current month and <= 2 of the next. This means that events late in the month won't appear.

This could be a design issue, and I'm open to the idea that we don't want to hold that much data at once. If we don't want to add all that extra data, the current design works fine.

**Describe the solution you'd like**

All of the events in the next month should appear, however many weeks into the future that is.

**Describe alternatives you've considered**

Keeping the current behavior, which does include much of next month, even if it's not all of it.

**Additional context**

In theory, changing this should be as simple as modifying the default value for `Calendar.Store.num_weeks` from 6 to something like 8.
In cases where people use a lot of events, though, this could lead to a lot of memory and probably tons of iteration loops in setting up the agenda view. For that reason, maybe it's better to restrict the number of events that we can hold.

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