issue_templates: Redirect feature requests

The existing Feature issue template is tailored for contributors who understand the process, have insight on benefits and drawbacks.

In practice, that's a minority of the people how use the template. For everybody else, the template appears to encourage submission of features requests without previous community discussion. This often results in poor quality issue descriptions which lack actual use cases and naïvely assert there are no drawbacks. Poor quality issues end up being closed, which sends negative vibes, feeding frustration and unproductive behaviors.

Aiming to avoid the negativity trap and promote community engagement, point people in the direction of Discourse and the place to discuss ideas. Also, repurpose the template to get more useful information on the underlying problems motivating an enhancement or feature request.