[Feature] Add a tentative date field in Issue Template

I was going over some issues for various events and talks and thought that it might need a new field for the issuer where he/she can add a date of his/her choice.

Locate the repository's issue template and read how its existing fields are organized. Add the requested tentative-date field with wording that lets an issue author provide a date, and verify that the template presents it clearly when creating an issue.

Written by the indexing model from the issue text.