Replace core.secure with core.https

[Retr0-01]

Currently any code (such as api/shorten.ts and api/upload.ts) that uses the core.https env var doesn't work and always returns a not-secure link, despite users setting the CORE_HTTPS variable to true in their local config.

That's because CORE_HTTPS has been mapped as core.secure in the config parser instead of core.https which is what the active codebase uses. This PR renames core.secure back to core.https to restore functionality.

[Retr0-01]

Seems to be fixed at latest release.