OT Extension: Automated origin trial extension UI changes

[DanielRyanSmith]

This change adds the UI components for the new OT Extension process that allows the extension to be processed without intervention.

Screenshot examples

The "Request a trial extension" button will now navigate directly to a form to fill out extension information without a prereqs dialog.

The extension form field will display Chromium schedule date information based on the given milestone.

Clicking "Learn how this date is chosen" will display a dialog with an explanation.

After submission, the user will have created a new extension stage, and will be navigated to the feature detail page that now displays the extension gate chip.

The gate chip will display when the extension has been approved, and a new "Finalize Extension" button will be displayed.

Selecting the "Finalize Extension" button will display a dialog that asks the user to verify the end milestone, and change it if needed.

[DanielRyanSmith]

@jrobbins This PR can't be merged without some prerequisite changes made available on the OT side, but it is available if you'd like to review. 🙂