Warn that folder required by a filter can't be deleted

[gtandersen]

Describe the bug When clicking to delete a folder that has a filter directing email to it, there is no response in the app.

To Reproduce Steps to reproduce the behavior:

1. Go to https://runbox.com/app and log in.
2. Set up a filter that directs email to a folder via Settings > Manager > Filter.
3. In Mail, click the ... button to the right of the folder and select Delete.
4. Observe that no action is taken or error given.

Expected behavior A warning should be shown that the folder is required by a filter and cannot be deleted.

Desktop (please complete the following information):

• OS: macOS
• Browser: Firefox