The Related Activities field for Resources should be a multiselect input for NetlifyCMS

[jobara]

Describe the bug

The config for the NetlifyCMS interface for the "Related Activties" field for the Resource pages only allows selection of a single item. However, the front matter is actually expecting a list of items.

The resources.njk layout attempts to iterate over the relatedActivities.

To reproduce

Steps to reproduce the behavior:

1. Open any of the Resource pages from the co-design.inclusivedesign.ca admin page.
2. Attempt to add multiple Related Activities
3. Notice you can only add one
4. If you add one, notice that it is added to the front matter as text instead of a list

Expected behavior

Related Activities should be a multi select input and add a list of the selected items to the front matter.

Additional context or notes

The feature should behave similarly to the Related Guides