Fix bad URLs / update 404 page

[gregjd]

https://amreldib.com/blog/FixJekyllCaseSensitiveUrlsOnGitHubPages

[gregjd]

Pseudocode:

if 'state/' in URL:
    replace('state/(\w{2})', 'state'+'\1'.toUpperCase)
elif 'doc/' in URL:  # handle URLs that got a month/day added
    URL.rstrip('/')
    doc_list = [i for i in site.docs if URL in i]  # i.e. get doc/seattle-wa-2016-02-01 since it contains doc/seattle-wa-2016
    if len(doc_list) == 0:
        # show normal 404 page
    elif len(doc_list) == 1:
        # redirect to that page
    elif len(doc_list) > 1:
        # redirect to state or place page
        # or list these policies on the 404 page
# Only if I change the URL structure for places, as suggested in issue #44:
elif 'place/' in URL:
    redirect to URL.replace('place/', 'docs/')

Also: 404 page should now say to check out Browse or Search, not just Browse.