"n more..." misplaced when calendar opens maximized (fullscreen, without wingpanel)

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

## Describe the bug
If calendar opens in full screen (wingpanel closed or hidden, so it doesn't occupy 24px of the top of the screen), "*n* more..." appears in the middle of day's boxes rather than at the bottom.

[Screenshot of misplaced tags](https://i.postimg.cc/Wbj4k1bC/pantheon-calendar-1080p-adwaita-dark.png); note: event details scribbled out.

### To Reproduce
Steps to reproduce the behavior:
1. Close, hide, or have never opened wingpanel.
2. Open calendar (and maximize it).
3. Observe misplaced tags in some or all day boxes.
4. Open or unhide (and restrut) wingpanel
5. Observe tags pop into place.

### Expected behavior
"*n* more..." tags should always align to the bottom of day boxes, regardless of window size, shape, or location at any given time.

### Screenshots or screen recordings
[Screenshot of misplaced tags](https://i.postimg.cc/Wbj4k1bC/pantheon-calendar-1080p-adwaita-dark.png); note: event details scribbled out.

## Platform Information
- OS: Arch Linux
- Linux 5.3.7-1-ck
- GTK 3.24.1.1

Please check what applies:
- [ ] I'm using the latest version from git that I've manually compiled
- [X] I'm using the latest released stable version

## Additional context
Discovered because of using [wingpanel with autohide](https://github.com/elementary/wingpanel/pull/93). The calendar app shouldn't rely on wingpanel occupying the top 24px of the screen to get its layout right. Also, this is an example of wingpanel enforcing a difference between "maximized" and "full screen": On the one hand, hiding wingpanel reveals calendar's broken layout; on the other, the extra 24px give calendar an extra row of info per day box.

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/84934121-n-more-misplaced-when-calendar-opens-maximized-fullscreen-without-wingpanel?utm_campaign=plugin&utm_content=tracker%2F60388692&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F60388692&utm_medium=issues&utm_source=github).

Start by reproducing the layout with wingpanel closed or hidden, then compare the calendar at maximized and normal sizes. No source file or test is identified in the issue; done means the “n more...” tags remain aligned to the bottom of each day box in all described window states.

Written by the indexing model from the issue text.