browse: Add UI components to retrieve vault bundles (WIP)

Integration of new GUI components in order to cook and retrieve
bundles from the SWH vault:

  - add buttons to request the cooking of directory/revision
    bundles in relevant browse views: directory, revision

  - use modals to get user confirmation and provide optional
    email for download ready notification

  - add a new tab 'Vault' in the main browse interface to
    report the status of cooking tasks and offer download
    links once they are available

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