Add support for #direct-call links

[ShGKme]

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Add support for #direct-call links. Used in notifications to jump into the call.

[Antreesy]

Read a bit, see two ways here:

• easy: custom popup (see sub-issue) should allow to configure action buttons from notification object (however we'd like it to be)
  • should be configurable (to disable and use native OS notifications only)
  • every OS styling?
• hard: use Service Workers - they allow to add action button to native system notifications
  • not supported by Firefox / Safari (shouldn't be the case for Desktop client, but blocks implementing in Web)
  • requires upstream changes at Notifications app

[ShGKme]

• Closed by feat: call notification popup (callbox) #868