Allow editing description for existing meetings

[armccoy]

Skips "new activity" check to allow updating meeting descriptions for existing and past meetings.
If necessary, a further edit could skip the "new activity" check if the meeting already exists AND if the meeting date/time hasn't changed.

[jrchamp]

Things to test:

• Edit only the description and make sure it successfully displays in Moodle
  • This must be successful for this PR to have benefit.
• Edit the description and the start time, setting the start time to a different time that is in the past
  • What is the error that occurs?
  • Does the description save in Moodle?
  • Does the description save in Zoom? If not, do we care?

[sgrandh3]

Tested the changes for also recurring meetings along with past meetings. The instructor can change the description for existing meetings

[jrchamp]

Thanks @sgrandh3 for testing!