mod_invites_page: Change client selection button text from 'Install' to 'Select' by default This also allows specific clients entries to override the text via the 'select_text' field. Rationale: 1) users may already have the software installed, we still want them to select it anyway for the tailored setup experience. 2) some clients (e.g. web clients) are not really "installed" so the text was misleading

This module adds a command to `prosodyctl` for listing users.

``` sh
prosodyctl mod_listusers
```