Invalid url for valid usernames

According to specification [4.3.1](https://spec.matrix.org/v1.6/appendices/#user-identifiers), a single `/` is a valid localpart for a user identifier.

Matrix.to shows that I can use `@localpart:domain`, and in fact `@name:host` does work, but writing `@/:domain` only shows "invalid URL" dialog - even when I explicitly write it in the text field and press "Create link", which I think is bad UX (should say that the name is invalid).

Also, according to the matrix specification [4.3.1.1](https://spec.matrix.org/v1.6/appendices/#historical-user-ids), clients MUST accept localparts that have any ASCII printing characters except `:`.

Start by reproducing the issue in Matrix.to's text field and the "Create link" flow using the valid localparts described in the Matrix specification. Inspect the URL and username validation entry point; done means valid usernames such as `/` are accepted and invalid input receives an appropriate name-validation message rather than an "invalid URL" dialog.

Written by the indexing model from the issue text.