closes #7622

closes #7617

closes #7618

closes #7621

closes #7616

- Removed posts and non contacts from other's data
- Collections are exported in batches to lower memory footprint
- In base exporters create User object instead of keeping instance because it caches all associations

closes #7627

It is not included into the main test suite, because it has :performance tag.

One can run this test using command:
$ bin/rspec --tag performance spec/integration/export/memory_usage_spec.rb

This test creates additional fixtures set to speed up repeated runs.

Main purpose of fixtures for us is to speed up test runs. Rebuilding
fixtures each run makes them pointless. They will be rebuilt automatically
if some of "files_to_check" are changed, so we're protected from the case
when fixtures are outdated and don't load properly.

It was possible to run migration locally without providing old private
key. This way migration was performed but not dispatched, which obviously
leads to desynchronization of the federation state so let's validate sender
before performing any actual actions.

closes #7558