use minimal app_config for tests
reload app config before each test

(complements 21bef8e304d05025b72b39e790abc7e8a4f4f5e1)
fix error with missing config option
better re-use of accept_invitation form

(complements 083d1dcb6d7ab9c03f11c74ea30a60f294db7f43)
improve signup