Notify organization members when transparency mode or monthly announcements are updated

[polypixeldev]

Summary of the problem

Currently, any manager can make changes to organization settings, and other members of that organization will not know that a change was made unless they are told by the person who enabled it.

For most settings like mission statement or contact email, this is fine - we don't want to be spamming members every time something minor is changed. However, transparency mode is something we want to make sure all members are aware of, and monthly announcements can be confusing if members are not aware that they are enabled.

Describe your changes

Adds 4 new emails to EventMailer:

• transparency_mode_enabled

• transparency_mode_disabled

• monthly_announcements_enabled

• monthly_announcements_disabled

These emails are triggered using callbacks on the Event and Event::Configuration models, and are sent to all members of an event.