Better security default for system account

[RLejolivet]

Fix #179

• "system" account password is randomized and shown in logs at application startup when it is activated
• Default is not to always activate the "system" account. Instead, it is only activated if there are no actual account created with enough privileges to grant more privileges, that could be used to recover back-office access (at least by granting itself more privileges if needed).

Note that it is possible to override the password and system account activation to recover previous workflow, but it must now be a conscious decision.

Also note that looking for a user with enough privileges to decide if the system account should be enabled will slow down startup somewhat, it is still recommended to explicitly disable system account when not needed.

[coveralls]

Coverage increased (+0.3%) to 53.262% when pulling f51200cfc89b612aaa53bdf207261e7799671be8 on RLejolivet:system-account-default-password into 13768d24ac1bcbc3347dc4bf5d41247484a7c1ab on blossom-project:master.