fixes #4353

* Create migrating_to_new_device.md

* Add link to Readme for further information.

* Added some info

* Make separate backup guide and integrate Readme information with new guides.

Co-authored-by: Licaon_Kter <licaon-kter@users.noreply.github.com>

fixes #4321

fixes #4338

fixes #4351
fixes #4261

...but why was that function created elsewhere if here you just compare this?

At some point a refactor changed this check from checking that the quicksy
domain itself is talking to you, to checking that anyone using quicksy is
talking to you, which breaks the notifications from strangers setting for
quicksy users.