Rename Upload::Base to Upload::Importer
Bump version with the Base -> Importer rename
New #all_issues method (used by Export plugins)
Import / Result - for an :id field so we can populate the plugin_id field
New Uploads::Base for common upload functionality
Content Service - take responsibility for caching existing Issues

Instead of having each plugin repeat the patter of querying the DB and
keeping a cache of "already in the project" findings, do it from the
Framework.

Benefits:

* Less repetition in each plugin's code.
* We can do smarter stuff on the RulesEngine side of things.

Also, inject the `plugin` and `plugin_id` fields into each incoming
Issue.
Import plugins - keep a double hash of plugins/filters
Bump version to 3.2
Extract Plugins::Base to a separate file