feat(xo-web): initial PWA manifest of Xen Orchestra 6

[MrGrymReaper]

Description

Implements the initial PWA manifest as per past request for PWA support quite a while ago, made by myself so decided to do the implementation myself. This implementation will aid businesses as with additional work after implementation of other parts of PWA by the community. The PWA specification can then enable the Xen Orchestra to be installed as an app via PWA on devices home screens. Thus business devices and ones involved in BYOD (Bring Your Own Device) based businesses and individuals on the System Administration team would then be able to easily and efficiently manage the XCP-ng and Xen Orchestra stack. This would be able to achieved through being able to click the icon on the home screen as well as later get notifications, with the Xen Orchestra icon and even when the window isn't open.

Checklist

• Commit
  • Title follows commit conventions
  • Reference the relevant issue (Fixes #007, See xoa-support#42, See https://...)
  • If bug fix, add Introduced by
• Changelog
  • If visible by XOA users, add changelog entry
  • Update "Packages to release" in CHANGELOG.unreleased.md
• PR
  • If UI changes, add screenshots
  • If not finished or not tested, open as Draft