Replace tinymce with tiptap

[bastianjoel]

resolves #1046
resolves #3311

TODO:

• Change marked text color
• Images
• Text color within marks gets discarded when parsing from html
• TinyMCE output compatibility
  • Marked text (TinyMCE: surround with span, style attribute with background-color)
    • Keep the background color
• iFrame / Replace topic editor
• Mso List start number
• Table styling inside editor display borders
• Menu layout should be in one line

[Elblinator]

If bullet points are copied with office 365, then the bullet point is a special symbol but not recognised as a bullet point

Here a screenshot:

The upper point is an actual bullet point
The lower point is the "bullet point" which was copied

[MSoeb]

Info: Editor should have a the possibility to edit the text via source code. Just like the TinyMCE one.

[bastianjoel]

I added the functionality but we might need to discuss the use cases for it as Prosemirror/Tiptap do not work with HTML internally and therefore only support tags and attributes that were specified beforehand.
We need to test this but I assume that this might lead to unexpected behavior for some users. Restricting that feature for motions might also give us some benefits for the diff algorithm as it might makes editor output more predictable.

[MSoeb]

When old content with images is opened and saved with the new editor, two problems occur.

1. the image changed it's position.
2. if the source code is also opened and closed with OK, the image is reduced in size.

Both cases must not occur as they change old content.

For Repro please contact me. Need to show it on a specific instance.

The change request is done.