Improvements for OOD and terminal MOTD and announcements

Our process for updating MOTD and announcements in OOD and at the terminal are ad-hoc and error-prone. Here are thoughts on improvements.

The goals are to have a single definition, document change procedures, streamline updates, and to fix an issue with the README file of https://gitlab.rc.uab.edu/rc/rc-ood-message not allowed to have a .md suffix, making it hard to read in GitLab.

Success looks like having a working README in the repo, a template for announcements, and a clearly documented procedure for changes to announcements and the MOTD, as well as automated updates across login004 and login005.

Proposed changes:

• Change OOD config:
  • Change MOTD_PATH=/etc/ood/config/motd/
• Change login005 image:
  • Change pull directory to something other than /etc/ood/config/announcements.d/. Should be on network filesystem.
  • Symlink the above two paths to point to wherever we pull.
• Change login004 image:
  • Use same cron job as login005 image to pull from repo to same directory
  • Symlink terminal MOTD to MOTD from repo.
• Change repo:
  • Add announcements.d directory with an announcement.md.template as a starting point.
  • Rename README adding .md suffix.
  • Adjust README to indicate announcements must go in the subdir.
  • Add to README that we need to compromise on markdown syntax to make MOTD readable at the terminal. Tables are OK. URLs should be "bare" links (no brackets). Section headings are OK, but use additional whitespace to help them stand out as needed.