Allow all of the URL schemes that Firefox allows

[aaronraimist]

This PR allows users to click on links using any of the URL schemes that Firefox allows + ftp (which Element already allowed) and sftp. Using the same list as Firefox was chosen arbitrarily but if Firefox allows them then they are probably safe enough to allow links to.

This PR implements what my comment in #6388 asked for

I think element-hq/element-web#8720 (comment) should be revisited. Yes linkifyjs won't linkify schemes it doesn't support but when a scheme isn't on this list then even an explicit link using will not work. It seems like Element should probably just allow all the same schemes that Firefox allows https://developer.mozilla.org/en-US/docs/Web/API/Navigator/registerProtocolHandler.

[germain-gg]

It seems sensible to allow those links. The safelisted schemes are available on https://html.spec.whatwg.org/multipage/system-state.html#custom-handlers

The only two listed here that are not marked as safe in the HTML spec are ftp and sftp. I wonder whether we should remove it as Firefox removed FTP support recently https://blog.mozilla.org/addons/2021/04/15/built-in-ftp-implementation-to-be-removed-in-firefox-90/

[aaronraimist]

The only two listed here that are not marked as safe in the HTML spec are ftp and sftp. I wonder whether we should remove it as Firefox removed FTP support recently https://blog.mozilla.org/addons/2021/04/15/built-in-ftp-implementation-to-be-removed-in-firefox-90/

I have no strong feeling either way. It does look like they are in the process of being safelisted though whatwg/html#6584. Browsers won't have FTP built in but they still want to allow people to open their FTP client of choice.

[germain-gg]

Let's go for the extended URL schemes! Thank you for suggesting that change 👏

[github-actions]

Here's what your changelog entry will look like:

✨ Features

• Allow all of the URL schemes that Firefox allows (#6457). Contributed by aaronraimist.