Refs #12636. Add doc. Add unit tests. Fix some bugs with 'single tick' replacement. New: double ticks in attributes are replaced by ''' - this guarantees that there are no more single ticks saved and the code is valid to be reused as variable inside QFQ Report.

Refs #12636. Add stored procedure. Add OnString::escapeSingleTickInHtml() incl. unit test. Next step: use  escapeSingleTickInHtml().

Refs #12615. Implements silent HEIC/HEIF conversion to png. Write a note in realease notes, to install 'libheif-examples'.

Refs #10754 apply partial fix to Dockerfile which is from Commit d9d69bb5. Commit d9d69bb5 was rejected due to  "rundumschlag", see ticket.