Add announcement feature to the website.

[prateekj117]

Along with notifications feature, there should also be announcements which rather than being targeted to certain people can be made publicly. And rights of making announcements should be given to Developers and Dean.

[prateekj117]

So I think the Announcement's Entity would be pretty much same as Notification's Entity, expect that it won't require recipient_channel and channel_type as announcements will be global. Also, I am thinking of keeping the automated and scheduled columns so that we can make automated announcements of known holidays.