Update `create_organization` condition to check application setting

Follow-up to !126225 (comment 1509231817)

In app/policies/base_policy.rb#L62 we temporarily set the can_create_organization condition to true so it can be used to setup routes. We should update this condition to check an application setting similar to what the can_create_group does.

• can_create_group condition - app/policies/base_policy.rb#L58
• can_create_group method - app/models/user.rb#L68

Implementation guide

• New database column in application_settings - can_create_organization, default true, not null
• Update the policy above

Mockup:

Not in scope

We decided to not implement individual user based can_create_organization

• New database column in users - can_create_organization, default true, not null
• Similar code to can_create_group in various places (See git grep can_create_group)