Update HB.Session.signIn to modern standards

 * Update signIn to use ic.ajax (and leave redirecting as an exercise for the reader)
 * Remove getAuthToken, signInWithOptionalRedirect (lol) and signIn aliases
 * Change url for signIn to /sign-in to match the signOut path
Fix validation of usernames on the settings page
better support for various youtube links
allow youtube videos to specify timestamp too. Closes #263
Merge pull request #283 from vevix/add-better-youtube-support

Better support for YouTube embeds