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`.
Merge pull request #254 from nidico/feature-show-interactive-debugger-link

Show link to weberror interactive debugger in debug mode