Update pin widget flow

[krschau]

Summary of the pull request

Accommodate the updated pin widget flow from microsoft/devhome#2110. Widgets are selected in the Add widget dialog and configured inline on the Dashboard, similar to how the Windows Widget Board does it.

References and relevant issues

Detailed description of the pull request / Additional comments

• Since we don't have to communicate to the host that we're in configuration mode anymore, it is removed from the Data json.
• Since there is no longer a different between "add" configuration state and "edit" configuration state, we can always show the Save and Cancel buttons in the Configuration template, although we only enable Cancel if there was once a valid state we can return to.
• Change UpdateActivityState() to use override keyword instead of new. By hiding the higher level method, the wrong one was being used. In the old flow, it happened to do the right thing, but wouldn't in the new flow.
• The Review Requested widget does not have a configure state, so throw an error if we request it.

PR in Draft until we get screenshot assets.

Validation steps performed

PR checklist

• Closes Update widget pin flow #311
• Tests added/passed
• Documentation updated