[Start DM] Look for an actual account in case of email address

[giomfo]

Your use case

When I open the contact picker to start a new DM, and type an email address, Element-Web doesn't check whether an actual account is discoverable for this email, whereas I configured an identity server in my profile.

Element-Android triggers a lookup request to replace this email address with a potential actual account

There is multiple advantages to discover user on email here:

• the end user has the confirmation their contact has a matrix account
• we worked on a new flow to create DM only on the first message Only create DM room when first message is sent #14736, select a matrix id is more powerful than an email in this case
• prevent the application from creating a new DM whereas one is already created for this user

Have you considered any alternatives?

No response

Additional context

The same enhancement is requested for Element-iOS: element-hq/element-ios#6497

[t3chguy]

Hmm this seems to have regressed #2933

[weeman1337]

Giving this a major severity, because you end up in having multiple DMs with the same person.