Admin can allow agents to modify announcements in customer portal

Following on from JSD-98

The base functionality has been added, so that a JIRA Administrator can set an announcement for the Help Center / My Requests page. And a JIRA or Project Administrator can set an announcement for the portal level.

This is to allow the Administrators to give permission to agents to modify these announcements.

Things to decide:

• Are agents only given permission to edit projects they are agents for, or can they also be allowed to edit the global Help Center / My Requests announcement?
• How will this look in the interface? Currently these settings are behind admin permission checks, so a display will be needed that allows agents to change the announcement but no other settings.