Previously, `Content-Disposition` was always set to `inline`, even for
HTML, which means that XSS could be easily acheived by uploading
malicious HTML and getting someone to click on the Matrix HTTP API link
for that piece of media. Now, we have an allowlist of safe values for
`Content-Type` that use `inline` while everything else defaults to
`attachment`, including HTML and SVG, which prevents XSS.
We also set the `Content-Security-Policy` header because why not.
A `set_header_or_panic` function is introduced to do what it says in
case Ruma begins providing better or worse values for the relevant
headers in the future. The safest way to handle such a case is simply
to panic.
This allows us to use the `ruma_route` convenience function even when we
need to add our own hacks into the responses, thus making us less
reliant on Ruma.
This change is fully automated, except the `rustfmt.toml` changes and
a few clippy directives to allow specific functions with too many lines
because they are longer now.
It comes with a bunch of new lints (yay!) so I fixed them all so CI will
keep working.
Also apparently something about linking changed because I had to change
the checks for deciding the linker flags for static x86_64 builds to
keep working.
