Unread-PMs count is blank if too big

The count we show on the "people" icon that leads to the PMs tab can look like this: I.e., there's a little bubble there… but instead of having a number inside, it's just blank, and so it looks broken.

This happens when the count is 100 or more. Specifically:

• I got that symptom when logged in as a test account that had 139 unread PMs.
• Then I went and read the messages one by one from a different client. The symptom stayed the same as the number of unread PMs went down to 100.
• As soon as the number became 99, it showed up properly:

I suspect what's happening involves the fact that the number is too long to really fit in that space.

Instead, when this count is big, we should probably just say something like "99+".