Draft: Add a new hyperkitty.settings module that defines all the settings.
This adds a new module, hyperkitty.settings, which includes the default values of all the settings that Hyperkitty currently uses.
This new module makes it such that all the settings are defined
in a single place and has consistent defaults instead of having a
default value specified with every .get()
call.
It also helps with documentation purposes of the application.