Add a general issue template

[rachellawson]

We should add a template that is more generic than the one for specifications.

I suggest we get inspiration from other projects and here is the one from Drupal, as detailed at https://www.drupal.org/docs/develop/issues/fields-and-other-parts-of-an-issue/special-issue-summary-templates/issue-summary:

Problem/Motivation

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet