MAIL_URL was overriden with database info all the time.
Now if MAIL_URL exists is not overwritten and if neither MAIL_URL nor
exists valid admin panel data MAIL_URL is not set.
MAIL_FROM was ignored. Same behaviour, env variable has bigger priority
than database configuration.
On both cases, althrought environment variable is set, updating admin-panel
mail settings will load new info and ignore the environment variable.
Remove some code that is not needed anymore
Only invited user can register in strict mode,
Set mail server in admin panel,
Switch strict mode in admin panel,
Invite people to system in admin panel
