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 writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.