add reachability tests

[alee]

see 69afdbb4d044dba395778437f87736cf70003622 for more context and a bug that bit us

Should hit all programmatically generated pages (in addition to wagtail pages)

• [ ] wagtail pages
• [ ] feeds
• [ ] codebase detail
• [ ] codebase list
• [ ] modeling frameworks
• [ ] journals