Update Semester Management Warning Styling

Issue Description: When a user clicks Administration and selects Semester Management from the dropdown, they are taken to an interface where they can add a semester. A red caution message informs the user that the most recent term automatically becomes the active term. This message currently appears too strongly as an error and uses the same UI style as a flash message.

Update the caution message to use a yellow warning style and display it as a persistent informational warning rather than a flash message.

Expected Behavior:
The caution about the most recent term becoming the active term should appear as a yellow warning and should have a distinct UI design from flash messages.

UI: