Describe the bug
Comment character count goes to negative if emojis are used after exceeding the limit

To Reproduce
Steps to reproduce the behavior:

1. Go to a meeting (or any commentable resource)
2. Add a long comment until "0 characters left" id displayed
3. Add an emoji
4. See "-4 characters left"

Expected behavior
Emojis cannot be added after exceeding the limit

Screenshots

Stacktrace
If applicable, add the error stacktrace to help explain your problem.

Extra data (please complete the following information):

• Device: [e.g. iPhone6, Desktop]
• Device OS: [e.g. iOS8.1, Windows 10]
• Browser: [e.g. Chrome, Firefox, Safari]
• Decidim Version: [e.g. 0.10]
• Decidim installation: [e.g. MetaDecidim]

Additional context
Add any other context about the problem here. For instance, add Metadecidim link.