FIX: Pull hotlinked images even when edited by system users (PR #9890)

Previously the pull hotlinked images job was skipped after system edits. This ensured that we never had an infinite loop of system-edit/pull-hotlinked/system-edit/pull-hotlinked etc.

A side effect was that edits made by system for any other reason (e.g. API, removing full quotes) would prevent pulling hotlinked images. This commit removes the system edit check, and replaces it with another method to avoid an infinite job scheduling loop.

Context: https://meta.discourse.org/t/thumbnail-generation-markdown-rendering-issue/152701/10

GitHub

Requires https://github.com/discourse/discourse/pull/9897

Is it possible to check that the jobs queue is empty instead?