ConversationView: fix last_status.account being empty, fix current user being included in group conversations

Done via the following command:
git diff fcd5dd25 --stat --name-only | xargs sed -i '/Pleroma Authors/c# Copyright © 2017-2020 Pleroma Authors <https:\/\/pleroma.social\/>'

[#1304]. Post-merge fixes. Merge branch 'develop' of git.pleroma.social:pleroma/pleroma into 1304-user-info-deprecation

# Conflicts:
#	CHANGELOG.md
#	lib/pleroma/notification.ex
#	lib/pleroma/user.ex
#	lib/pleroma/user/info.ex
#	lib/pleroma/web/admin_api/admin_api_controller.ex
#	lib/pleroma/web/ostatus/handlers/follow_handler.ex
#	lib/pleroma/web/ostatus/ostatus.ex
#	lib/pleroma/web/salmon/salmon.ex
#	lib/pleroma/web/websub/websub.ex
#	test/web/admin_api/admin_api_controller_test.exs
#	test/web/federator_test.exs
#	test/web/mastodon_api/controllers/conversation_controller_test.exs
#	test/web/ostatus/ostatus_controller_test.exs
#	test/web/ostatus/ostatus_test.exs
#	test/web/salmon/salmon_test.exs
#	test/web/websub/websub_test.exs

This reverts merge request !1737