Forbid input with letter buttons after 16 symbols had been input

[queleok]

Describe the bug
Right now if one enters 16 symbols into the input from the keyboard, there will be no way to input more the same way. However, pressing unhighlighted letter buttons would still add the letter to the end of the input word. This is simply wrong.

To Reproduce
Steps to reproduce the behavior:

1. Focus on the input
2. Add 16 a letters
3. Press any unhighlighted letter button
4. Observe it highlighting and being added to the end of the input word

Expected behavior
No highlighting and no addition of a symbol