HTTP header improvements (#294)

HTTP header improvements (#294)

- Drop Expect-CT

Expect-CT has been redundant since 2018 when Certificate Transparency became mandated and required for all CAs and browsers. This header is only implemented in Chrome and is now deprecated. HTTP header analysers do not check this anymore as this is enforced by default. See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Expect-CT

- Raise HSTS to 2 years and explicitly preload

The longer age for HSTS, the better. Header analysers prefer 2 years over 1 year now as free TLS is very common using Let's Encrypt.
For HSTS to be fully effective, you need to submit your root domain (domain.tld) to https://hstspreload.org. However, a requirement for this is the "preload" directive in Strict-Transport-Security. If you do not have "preload", it will reject your domain.

- Drop X-Download-Options

This is an IE8-era header when Adobe products used to use the IE engine for making outbound web requests to embed webpages in things like Adobe Acrobat (PDFs). Modern apps are using Microsoft Edge WebView2 or Chromium Embedded Framework. No modern browser checks or header analyser check for this.

- Set base-uri to 'none'

This is to specify the domain for relative links (`<base>` HTML tag). pleroma-fe does not use this and it's an incredibly niche tag.

I use all of these myself on my instance by rewriting the headers with zero problems. No breakage observed.

I have not compiled my Elixr changes, but I don't see why they'd break.

Co-authored-by: r3g_5z <june@terezi.dev>
Reviewed-on: https://akkoma.dev/AkkomaGang/akkoma/pulls/294
Co-authored-by: @r3g_5z@plem.sapphic.site <june@terezi.dev>
Co-committed-by: @r3g_5z@plem.sapphic.site <june@terezi.dev>