Can't invite users by mxid

**Is your feature request related to a problem? Please describe.**
If a user doesn't share a room with you and isn't in your contact list (and haven't registered with an identity server either, I suppose), it is not possible to invite that user to a room via their mxid.

This is because on the "Invite Users" screen, typing in someone's mxid doesn't give you the option of selecting that mxid unless "Known Users" or "Suggestions" find a match for it.

This means that the only way to initiate a conversation with such users is to use another client.

**Describe the solution you'd like**
The "Invite Users" screen should allow sending invites to an arbitrary mxid, even if the client can't verify whether the mxid is truly valid. Element Web allows this, and the old Riot Android did as well.

**Describe alternatives you've considered**
To deter sending invites to invalid mxids, a warning dialog could appear for any mxids that can't be verified. The user should be to make the ultimate decision to send such invites anyways.

**Additional context**
Element Web has [a similar issue](https://github.com/vector-im/element-web/issues/15264) where pasting someone's mxid will fail if a matching user can't be found, but this can be bypassed by typing it manually. As far as I can tell, this is the only way to invite an unfamiliar user with Element-branded clients. (I haven't tried Element iOS.)