Adding CSS rule for Markdown Blockquotes
Adding CSS rule for Markdown Blockquotes

Using a vertical bar next to the text to indicate a blockquote.
Using lighter text color as well (similar to github's blockquotes).

resolves #2064

Signed-off-by: Sebastian Bachmann <aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d@reox.at>

juliushaertl/deck,nextcloud/deck,nextcloud/deck,nextcloud/deck,juliushaertl/deck,juliushaertl/deck