Active Sessions 0.2.0

A sign-in now ends after a set time by role, kept on the server: everybody 12 hours, the roles you choose (default: global admin) 7 days, both in the module's settings (0 = no limit). A sign-in past its time is ended on the person's next request (the login page says why) and, every few minutes, its session row is deleted, so pages outside Omeka that read the session table see it gone too. Sign-ins from before the upgrade start their clock when first seen, so upgrading signs nobody out at once. The sessions page says the rule at the top and when each sign-in ends. Upgrade from 0.1.0: Modules, Active Sessions, Upgrade (adds one column). See CHANGELOG.md.