Groups: Implement "Announce Event" email for group followers

[mariobehling]

For future events group owners should have the option to announce an event via email to group followers. There are different ways to make this user friendly, but for now the first step to implement this in the UI would to add an "Announce" button into the my groups view page.

This should work as follows:

1. Only future events can be announced.
2. Events can only be announced once. After the initial announcement the user should not see the button anymore and instead see the info "Announced".
3. If the user has not announced the event yet clicking on the button should show a pop up. "This will announce the event to all followers of that group. Do you want to announce the event now? Yes / Cancel"
4. If the user chooses "Yes", a) Show a pop up box with an email (similar to the email boxes sent to speakers, but not editable) b) Show buttons "Send / Cancel". If the user clicks "Send" send an email to all followers. Include the following information:

• Name of event
• Date
• Location
• Link to public event page
• Event description
• You're getting this message because your [app name] account is connected to this email address. You can unsubscribe from similar emails from this group by unfollowing the group here. (add link to public group page here}

5. If the organizer has announced the event already show "Announced" in the same place where the previous "Announce" button was placed.