Fixes #10402: add to_bool function for converting to boolean.

This commit adds a to_bool function for casting values to boolean
as well as an example usage.

http://projects.theforeman.org/issues/10402

Fixes #5568 - Improves performance of trends:clean rake task

Perform trend counter dupe counting within the database rather than in the Ruby code.

Fixes #8727 - DHCP validation does not fail on discovered leases

fixes #9775 - always load Encryptable when key's missing, log runtime warning

a59972c3 causes Encryptable to be loaded before the encryption_key.rb
initialiser and the majority of the class was skipped as the key was undefined.

Now Encryptable always loads, but logs at runtime if the key is unavailable,...

Fixes #7505 - Changed the way trends are stored in the DB. Now they are stored as intervals

Fixes #9593 - replace .all.map(&:<attr>) with .pluck(:attr)

fixes #8331 - provide some tools for filtering mail

Refs #3809 - Remove various small cops

Refs #3809 - Remove cops for empty lines

fixes #9212 - using headless browser in order to run Capybara tests