Remove /validuser endpoint now that the Trac wiki is gone.
cookies: Make over18 cookie not IP-hashed.

The over18 cookie's IP hashing behavior is of dubious use and is not at
all compatible with efficient CDN caching.
cookies: Gracefully handle invalid encrypted blobs in _options.

This fixes a TypeError from b64decode when the input is bad. This patch
is needed to gracefully survive new-style cookies during the rollout
period.
cookies: Don't encrypt the unlogged-user _options cookie.

This cookie is part of the CDN cache key. Since the encrypted blob uses
a random salt, this essentially makes requests with the cookie
unchacheable. The cookie does not contain any sensitive data (just a
list of languages and a boolean True/False). Decrypting and normalizing
the order of params means that the cache key will be more useful. We
also get a fair reduction in size of the cookie.