Update "Manage Lesson Resources" saving actions with clearer language

[marcellamaki]

Observed behavior

During the Uganda training, coaches reported confusion about clicking the "close" button during the Manage Lesson Resources flow. Not always super intuitive to click on "Close" after desired exercises are selected. It seemed a little intimidating to hit close instead of something like “Save” or “Next.”

Expected Behavior

Replacing the "Close" button with something like "Save" or "Continue" (available in commonCoreStrings) might make the workflow clearer, even though the resource is technically saved as part of the checkbox action.

User-facing consequences

Users being unsure of the next step of the workflow

[rtibbles]

Fixed in #9516