mod_invites_page: Remove preauth URI button This button is incompatible with the majority of XMPP clients around, yet based on feedback from users, many are drawn to click it when they have any XMPP client installed already. In the case where the user already has software installed, we would prefer them to select it from the software list so they can follow the setup process suited to their specific client (we already track which software supports preauth URIs). If their client is not listed, they can still use the manual registration link instead.

This module logs events where more than a certain amount of CPU time was
spent.

``` lua
log_cpu_threshold = 0.01 -- in seconds, so this is 10 milliseconds
```

Uses the Lua
[`os.clock()`](http://www.lua.org/manual/5.2/manual.html#pdf-os.clock)
function to estimate CPU usage.