Truncate messages on load and add an option to disable it

[link2xt]

Messages are not converted to HTML when saving, but when loading.

There is a new API to disable truncation.

Closes #7718

[Simon-Laux]

I think to merge this we need corresponding UI PRs, at least in iOS long messages were problematic in the past deltachat/deltachat-ios#1634

[r10s]

moreover, we need to take care not to harm the existing, shipped ecosystem

[link2xt]

We need corresponding Android PR. For Desktop, iOS, Ubuntu Touch, possible solution is to keep truncating the message, but on load rather than when saving to the database.

[Hocuri]

@adbenitez since you created the corresponding issue: I think it makes sense to test this before merging so that we can changen it if it doesn't fit for unexpected reasons - so, would be nice if you can create a corresponding Android PR at some point (not too urgent, though, just to get this done at some point) :)

[Hocuri]

FTR, this PR just adds an API dont_truncate_long_messages() that can be called on startup to disable truncating. If the UI doesn't call this API, then the behavior won't change (unless there is some bug); whether they are truncated on loading or on saving is just an implementation detail in core.