Notification icons will now look pretty for admins

I found that only 2 icons were missing error & question. Calendar was
overly huge. I decided it would be best to use the icons that are
actually rendered on the dashboard where the announcement is visible
to users.

I also moved some business logic out of an erb template and into a
helper. The announcement layout in the admin area is now more inline
with how they are actually rendered on the dashboard.

Fixes CNVS-14421

Test Plan:

- Log in as an admin and go to an account's settings page.
- Click the Announcements tab and Click the blue "+ Add a New Announcement" button.
- Fill in all the details of the announcement, but leave the icon as the default, then click the Add Announcement button.
- Verify the announcement appears on the announcements list and the icon shows a Yellow warning sign with an exclamation mark.
- Now, create a new announcement and change the icon to something else, then click the Add Announcement button.
- Look at the icon on the announcement's icon on the announcement list. The icon should now show up.

Change-Id: Ie8c0ad64c5de5ab6ab76b90ff3e95712f71be8d6
Reviewed-on: https://gerrit.instructure.com/38609
Reviewed-by: Mark Severson <markse@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Janelle Seegmiller <jseegmiller@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>