Added FeedSource has_many Feeds association with migrations
Fast prototype of MainContentExtraction api integration
Added Feed#main_html_content and FeedSource#active,extractable_main_content columns.
Also added possibility to set FeedSource as extractable_main_content in the
prototype content extraction view.
Changed Feed#url to :text to try to resolve issue
Created Scheduler::MainContentExtractor and added to clockwork
with 5 minute period.
Added main_html_content show in FeedsController#show