Add system/mailer settings to db/seeds.rb

[Holmes98]

These are currently created during migrations, but we plan to remove this behaviour in #243. The values are left blank, as they need to be set manually anyway.

@tom93 suggested using an environment variable for the password; I didn't bother, but feel free to add that if you prefer.

Commit message can be changed if we decide not to go with #243.

[coveralls]

coverage: 37.131%. remained the same
when pulling 6d2bd1b on seed-mailer-settings
into 57b299d on master.

[bagedevimo]

IMO this is worth doing even regardless of migration changes, as it makes the whole thing more complete with just a db:seed

[Holmes98]

Done in #273.