fix code block newline, 15->20 max tagsize

[cloverich]

• Fix issue in code block formatting where multtiple saves without a code block edit would add extra newlines to code blocks. Caused by how mdast <> slate transformation was interpreting code_line elements, fixed with trial and error as opposed to a principled approach
• Extended max size of tags from 15-20 as it was already too small to support tags I am using on other platforms. Max size is also set as a guestimate, and not based on a principled approach