Improve sharing self when creating a DM

[nadonomy]

From recruited user testing— users struggled to understand they could share a link for themselves when creating a DM.

[niquewoodhouse]

Based on the idea this is a papercut so small changes

As it currently is for reference

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Stuck to bottom like Discord

At top of dialog content as a CTA

Tabs to keep content changes on this view down

As an extra line of copy in the intro

[t3chguy]

Any of the options are feasible, the tabbed approach is probably a tad bit more work

[niquewoodhouse]

On reflection, think this makes the most sense.

• Puts the matrix link underneath the existing form. If it goes at the top, I worry it might lead to more people using the link than the form, and there are known papercuts in that experience.
• Solves Improve starting a DM when no results are returned #17359 by putting something at the end of the list of results that points to the link

@t3chguy I tried using theme colour for the new things, I don't mind if the copy url field has its own pre-existing css for colours etc, we can just use that. Figma link