fix(richText): do not handle relative links without leading slash as router links

[ShGKme]

☑️ Resolves

• Fix Mailto in avatar overlay is wrong nextcloud/spreed#12516
• autolink supports relative URLs, for example, /call/ab34cd is resolved as a Talk route.
• It considers a link as relative if it doesn't start with https?://.
• However, call/abc or mailto:email@nextcloud.ltd also doesn't start with HTTP.
• The best is to only allow relative URLs with leading slashes /.
• Even the check for leading / is enough for explicitly adding a check for non-http schemes.

VueRouter handles such URLs according to the URL RFC, for example, being on /call/abc page, a link def is resolved as /call/def relative to /call/. We don't want to support such links anyway, so relative URL MUST start with a slash.

Alternative solution: add a leading slash / to relative links.

🚧 Tasks

• Check for non-HTTP schemes.
• Support relative links only if they start with /.

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 3️⃣ Backport to next requested with a Vue 3 upgrade

[nickvergessen]

Looks like it should fix it, but didn't test

[susnux]

/backport to next