[$40] Admin user cannot make announcements

[sdgun]

@atelomycterus - I fixed the permission issue below, but it seems that the New Announcement button is just adding a discussion, not an announcement.

Steps
Log in from TonyJ user
Go to https://vanilla.topcoder-dev.com/group/715
Click New Announcement
Create an announcement >> save

Show the below error:

[jmgasper]

Challenge https://www.topcoder.com/challenges/415aca7c-ba6e-4131-9d49-adf263d41792 has been created for this ticket.

This is an automated message for ghostar via Topcoder X

[jmgasper]

Challenge https://www.topcoder.com/challenges/415aca7c-ba6e-4131-9d49-adf263d41792 has been assigned to obog.

This is an automated message for ghostar via Topcoder X

[atelomycterus]

@jmgasper Please apply PRs:
#455
topcoder-platform/forums-groups-plugin#64
Thanks!

Testing

1. The New Announcement button should be displayed if user has 'Vanilla.Discussions.Add' and 'Vanilla.Discussions.Announce'.
   New Announcement button should add an announcement.
   

2. Announce an existing Discussion
   'Vanilla.Discussions.Announce' is required.