Show link to weberror interactive debugger in debug mode

Weberror allows for interactive post-mortem debugging. The respective
middleware has already been active in available previously.

This commit allows to expose a link to the interactive debugging session
from the HTTP500 error page through the configuration option
`adhocracy.interactive_debugging`.

See the [Pylons docs](https://pylons-webframework.readthedocs.org/en/latest/debugging.html#interactive-debugging).

I think this is very new-developer-friendly, so I propose to enable this by default (in debug mode).