Added smooth painting/erasure feature

1. Refactored some code (I splitted `$(document).on("click", ".day", function () {` into four functions: `activateDay`, `deactivateDay`, `isDisabledDay` and `tryToggleDay`)
2. Added the ability to hold down <kbd>Shift</kbd> to paint smoothly.
3. Added the ability to hold down <kbd>Ctrl</kbd> to erase smoothly.
4. Deactivated the tooltips when one of the keys mentioned above is pressed.

(I should have done multiple commits :zzz: )