Process message HTML before editing it

[dtdesign]

The HTML of a message is currently directly fed into the editor without the ability to make server-side modifications or adjustments. This requires us to duplicate some logic, for example, the <woltlab-metacode> to BBCode transformation, in the editor.

In addition such a pre-processing would allow us to apply settings like the image proxy for the editor too. Quoting a message that contains external images while the image proxy is enabled will implicitly leak the user‘s IP address when inserting the quote into the editor.