Support sending invite reasons from main invite flow

#### Is your suggestion related to a problem? Please describe.

Now that invite reasons are shown, it would be useful to have a way for non power users to send them with invites (i.e. without the use of the /invite command).

#### Describe the solution you'd like.

I would like for there to be a text box in the invite dialog to optionally add an invite reason, labeled "Add a message (optional)" or something similar.

It would be nice to be able to send a reason when starting DMs too, though I'm not sure how best to do that since the idea of having an intermediate dialog when starting DMs has been rejected in the past as creating too much friction. One potential solution that's been discussed in the past is to turn the "Direct message" button in peoples' profiles into a message box of sorts, similar to Discord. (see https://github.com/vector-im/element-web/issues/16300#issuecomment-774098678)

#### Describe alternatives you've considered.

Alternatively invite reasons could just remain a slash commands feature, if it's decided that it's too much cognitive load to add another text box to the invite dialog.