Add environment-specific seeds and load plugin-seeds

The commit adds a mechanism to load seeds that are specific for the
given environment. The seeds are stored in db/seeds/<environment>.rb
and should be written idempotent, to make sure, they don't
destroy/overwrite anything when being run multiple times.

The seeding now also loads the seeds for the plugins, which allows
plugins to add plugin-specific constants to the database.
Removed overwritten mail method and explicitely
removed html parts while collecting if the
Setting only permits plain text mails
Merge pull request #336 from opf/feature/rails3_rewrite_usermailer_format

Removed overwritten mail method and explicitely
removed the duplicate comment [skip ci]
Merge pull request #335 from opf/feature/rails3_enviromental_seeds

Add environment-specific seeds and load plugin-seeds