Remove trial banner for admins & viewers (#1308)

lib/plausible/auth/user.ex

lib/plausible/billing/billing.ex

lib/plausible/sites.ex

lib/plausible_web/controllers/auth_controller.ex

lib/plausible_web/controllers/invitation_controller.ex

lib/plausible_web/controllers/site_controller.ex

lib/plausible_web/templates/site/index.html.eex

lib/plausible_web/templates/site/new.html.eex

...grations/20210908081119_allow_trial_expiry_to_be_null.exs

test/plausible_web/controllers/auth_controller_test.exs

.../plausible_web/controllers/invitation_controller_test.exs

test/plausible_web/controllers/site_controller_test.exs