Update to Phoenix 1.3

[jbr]

This PR updates all dependencies to most recent versions except guardian. Updating to guardian 1.0 will be a separate PR. This touches a huge number of files because Phoenix 1.3 wants everything in a different place, and nearly every module is renamed. Some js formatting was changed because I run prettify by default, but I'm happy to revert this if it's a problem. Tests pass, most existing functionality works, and there are no compiler warnings. The one bit of functionality that I removed wholesale was the Slack OAuth strategy, because without https://github.com/ueberauth/ueberauth_slack/pull/16, ueberauth_slack depends on an outdated and incompatible version of oauth2.