Use secure flag for cookies

[bwbroersma]

Using insecure cookies is insecure, part of defense in depth.

abacus/backend/src/api/middleware/authentication/mod.rs

Lines 25 to 26 in 8746f80

	/// Only send cookies over a secure (https) connection
	pub const SECURE_COOKIES: bool = false;

Requires TLS.
Note: clicked the wrong template, but adding TLS might be an Epic.