display number of comments on submissions index page
ignore the db/dumps folder
implement counter cache for comments
implement comment counter cache, crush those n+1's
model tests for comments counter
Update counter cache for submission comments.

Was missing an `.each` after plucking the IDs. Also included dummy
models in the migration in case comments or submissions are removed
in the future.
Show comments link if one or more present.

Makes it easier to see which submissions have been commented on
and which haven't.

Also pluralized the comment count if greater than 1.
Merge pull request #105 from LaunchAcademy/total_comments

Total comments