$nr_of_rows and $pages_per_set where respectively set to 5 and 7
in several files.

Since $nr_of_rows was set to config->{posts_on_page} in
lib/PearlBee/Posts.pm, the obvious fix was to follow that example and
make those values configurable in the main config.yml.

Defaults are assigned at the application startup, so there is no need
to further check if the values are valid.

posts_on_page still has a number of issues:
- it should probably be named posts_per_page
- it is actually used different things, like limiting the number of
  users or comments shown in the dashboard

Further improvements include the addition of users_per_page,
comments_per_page, etc.