disable comments for "too old" episodes and enable spam reporting again

[ruderphilipp]

• Chaged episodes#show to disable comments if episode was published "too long ago" (added method in model and rspec tests) - this might be helpful to suppress spam comments on older episodes since most discussion goes on directly after publishing. Thus, I chose to set 3 months as a limit for new comments.
• Changed comments/_comment.html.erb to always show "report as spam" link and not only if no user available - I did this because newer comments cannot be reported anymore on railscasts.com since the authentication system with Github was introduced.

[ryanb]

Thanks for sending this pull request. Since I added GitHub sign-in the spam problem is not as bad. My plan is to add moderators to help clean up the existing spam because I still want to keep the comments open.