Add image processing and generate blurhash for server thumbnail (#19348)

app/models/form/admin_settings.rb

app/models/site_upload.rb

app/presenters/instance_presenter.rb

app/serializers/rest/instance_serializer.rb

app/serializers/rest/v1/instance_serializer.rb

app/views/admin/settings/edit.html.haml

app/views/application/_sidebar.html.haml

app/views/shared/_og.html.haml

config/locales/en.yml

db/migrate/20221012181003_add_blurhash_to_site_uploads.rb

db/schema.rb

spec/presenters/instance_presenter_spec.rb