FieldProcessor - provide a dummy implementation of :post_initialize

This avoids raising an error if the add-on writer doesn't provide its
own.
Dradis::Plugins::Base - memoize the :plugin_name
ContentService - manage validation errors by add-on writers

Do not trust the add-on writer to provide valid models, account for the
possibility of AR validation errors on creation.

Also, some times a plugin is too keen a provides too much data as
Evidence. Try to manage that via length restrictions and warnings to the
user.