Adding project tags to slowvote polls

Summary: T2628, project tags in slowvote polls

Test Plan: Open poll, edit, add project tags, save. Poll should show tagged projects and a relevant transaction. (transaction doesn't currently show up)

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9708