Set the preferred URL scheme in production to https

[AetherUnbound]

Description of Changes

We are seeing an issue where calls to url_for using _external=True for certain conditions are generating links to files that include the scheme, but use http for it instead of http. This is causing a mixed content warning with our certificate. I tracked it down to pages that are serving the default placeholder image for officers, e.g.:

OpenOversight/OpenOversight/app/templates/officer.html

Line 8 in 6cd9857

{% set image_url = officer.image_url | default(url_for('static', filename='images/placeholder.png', _external=True)) %}

.

This PR changes the PREFERRED_URL_SCHEME setting to https for production environments.

Tests and linting

• I have rebased my changes on main

• just lint passes

• just test passes