Fix "HIX score outdated"

[PeterNerlich]

Short description

Any action within the tinymce editor modifies the content, and should thus set the content to be dirty. At least, this is the assumption of tinymce. But there are actions that do not modify the content, like making a selection, or end up at a previous state, like adding a character back after accidentally removing it.

This PR fixes this problem by simply monkeypatching tinymce's isDirty() and setDirty() methods, always comparing it to a previous value and saving that on successful autosaves.
Evaluating the HIX score also sets such a value to use as basis for determining whether an earlier score is actually outdated or not.

Proposed changes

• Monkeypatch tinymce isDirty() and setDirty() to actually represent the content being different from the last saved state
  • tinymce provides this nice startContent property that is initialised to the initial content and then never used anywhere
  • We compare against that and set it after successful autosaves (using the actual content that was sent when the autosave was triggered)
• Have a similar value to compare whether the content is different from the last HIX evaluation and display the hints based on that

Side effects

• isDirty() behaves differently now (not relying on the isNotDirty property anymore) and setDirty() tries to keep isNotDirty in line with what the new isDirty() would probably return if no changes to this are missed (this might get out of sync when an autosave is performed)

Additional notes

• Where should I best put the monkeypatch?
• Do we want a monkeypatch at all? (It seems very logical to me in this case, but it still feels… dirty 🙃)

Resolved issues

Fixes: #2300

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[codeclimate]

Code Climate has analyzed commit 1b137d5 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 76.7%.

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[MizukiTemma]

Thank you for the PR 💪 It fixes the problem 🙂

But some side effect was found:

1. an unsaved warning will be shown when some changes are really made and the page is published or saved as draft. For example, add some words in a page and save it.
2. After leaving this unsaved warning by chosing "Verlassen" and saving the page several times, IntegrityError will be raised.

[PeterNerlich]

I cut out everything not related to this issue and put it in #2476

[MizukiTemma]

Thank you for the updates 🙂 Looks good!

[seluianova]

LGTM!