Projects 2.0: Allow commenting on cards

[alramalho]

Projects 2.0 has high potential, specially as a replacement for poor UXed tools like Jira.

Although, there is a core feature missing, that is comments on cards.
This is helpful for using Projects 2.0 as backlog, or simply as discussion panel.

Comments are a crucial feature, and I would love to see it implemented.
Thanks for all of your beautiful work!