Redirect to Checkmate error pages

[jon-betts]

For: hypothesis/checkmate#15

This removes ViaHTML's own error pages and instead relies on whatever Checkmate tells us to show to people. We do a temporary redirect there for the moment, as we don't have any indication of how long the block should last for.

This cannot be merged until checkmatelib has been updated with: hypothesis/checkmatelib#5. The version number in the requirements is therefore a guess.

Testing notes

• Check out checkmatelib as a sibling to this project
• In checkmatelib run: git checkout add-html-view
• In checkmatelib run make services dev
• Hack the requirements.txt to replace the checkmatelib line with -e ../checkmatelib
• Run make dev
• Visit a blocked page: http://localhost:9085/proxy/http://bad.web.app
• If you have the real rules loaded this should redirect (if not try bad.example.com instead)