Preparing release 2.2.20
Back to development: 2.2.21
Add text block tests.
Implement book text block based on ftw.contentpage text block.
Rename paragraphs to textblock in tests and docstrings.
Update block LaTeX to render BookTextBlock instead of Paragraph.
Change validation viewlet to validate new BookTextBlock instead of Paragraph.
Change target release to 3.0.0.
Add upgrade step for replacing Paragraph with BookTextBlock.