Add endpoint for fetching activity stats

This is secured by a token that is only accessible by the admin, and is
regeneratable on the /stats page.
Change activityStatsTokens id's to have more entropy
Fix problems found by David.
Do not inherit host environment when entering the Sandstorm outer container.

Many kinds of environment settings become invalid or problematic after a chroot (or pivot_root in our case). For instance, an LD_PRELOAD would now point at a directory that doesn't exist. More commonly, a LANG setting other than C or C.UTF-8 could be problematic as the locale files distributed it the Sandstorm bundle is taken from the machine where the bundle was built and probably don't support arbitrary languages.
Fix id(1) output parsing in non-English locales. Fixes #122.
Remove redundant seccomp rules, and some other style fixes. Fixes #98.
Make installer default to BASE_URL local.sandstorm.io instead of localhost.

This is important because the WILDCARD_URL will have to be *.local.sandstorm.io since there's no way to set a wildcard in /etc/hosts (which is how 'localhost' is defined). If these two URLs aren't under the same domain, browsers or extensions that block "third-party cookies" will break Sandstorm.

Fixes #78.
Improve error message when the request hostname isn't recognized.

The obscure error message here tended to confuse people who had merely set BASE_URL incorrectly in their sandstorm.conf.

Fixes #82.
Use isAdmin check in regenerateStatsToken
Merge pull request #125 from jparyani/stats

Add endpoint for fetching activity stats