fix pattern validation in event deletion
Event title is required to confirm event deletion in the dedicated dialog.
The pattern attribute of an input HTML element, used for this validation, is a regexp, not a string.
If event title contains parenthesis or square brackets, then the validation pattern does not match the title anymore.
All special characters of a regexp must be escaped in the event title to use it as a validation pattern.
This is the purpose of this PR.
Here is a screenshot of the bug :