Add support for inline images in editor

[StCyr]

fixes #2052

(based on branch enhancement/rework-duplicate-entries-for-tag-message since master doesn't work for me atm)

TODO

• ckeditor magic with a custom uploader that uploads to our temporary attachment storage
• a route that serves images of those temporary files
• logic that changes from the temporary URL source to a part when the message is sent

If someone wants to help me, he/she's most welcome 😃

[miaulalala]

@StCyr master should be fixed now 👍

[ChristophWurst]

Superseded by #6935 🚀

Thanks a ton for your effort @StCyr. It took us all a few attempts to get this feature done 🤞