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

• This PR makes significant changes to the customization template, removing the arrow button and Submit button. Instead, input validation is done on Save.
• Since we don't have to communicate to the host that we're in configuration mode anymore, it is removed from the Data json.
• CanPin is renamed to CanSave, a more accurate name.
• Pinned indicates the state that the widget has ever had valid configuration data, and canceling customization will return the widget to that data.
• Save and Cancel buttons are no longer labeled "positive" and "destructive" to give them normal styling.
• 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.

PR in Draft until we get screenshot assets.

Validation steps performed

PR checklist

• Closes Update widget pin flow #90
• Tests added/passed
• Documentation updated