Adds an option to restore activity bar defaults

[guerler]

This PR adds a panel button to the activity settings panel. Clicking on this button enables users to restore the Activity Bar defaults.

How to test the changes?

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[martenson]

We use that arrow icon for "undo" in other context, could we use labeled button with "reset" or something similar to clarify what it does?

[guerler]

I would say that this is consistent. The undo/reset is only used in the workflow run form where users can reset the default input value. Additionally there is a tooltip and a modal warning before resetting the activity bar settings. Nevertheless I added a label. This seems to work given that there is only a single header button available.