[Bugfix] Use safe nav operator to handle nil descriptions on announcement index page

[samudary]

Why

Prior to introducing the presence validation on Announcement#description, several announcements were created in production without a description. This broke the announcement index page because there were a couple references that depend on a description present.

What

Fixes #888

• Uses safe navigation operator when referencing a description on the announcement index page
• Casts the description length to an integer to conditionally render a truncated description and more link

How

N/A

Testing

N/A

Next Steps

N/A

Outstanding Questions, Concerns and Other Notes

N/A

Accessibility

Security

Meta

Pre-Merge Checklist

• Security & accessibility have been considered
• Tests have been added, or an explanation has been given why the features cannot be tested
• Documentation and comments have been added to the codebase where required
• Entry added to CHANGELOG.md if appropriate
• Outstanding questions and concerns have been resolved
• Any next steps have been turned into Issues or Discussions as appropriate

[samudary]

@maebeale Ready for review

[maebeale]

LGTM! Thanks! :)