Fix problems with displaying filtered changes

Commit a355b480ed146c49f18e7a5b9452e38b23764857 renamed the local
'filter' to 'filter_value', but not everywhere.  The places it
missed were mostly comparisons to strings, which now ended up
comparing the built-in function filter() to those strings instead.
Add Johan Herland to CONTRIBUTORS
Drop unused func usesExperimentalFeature and some related cruft
Hint about "criticctl CMD --help" in "criticctl --help"
Fix crash when anonymous user loads /showfilters?repository=smth&path=/

Also let path default to "/" if no such param is passed.