Add support for user data tags

[azasypkin]

Summary

Today, users are unable to group the content they create, including responders, certificates, trackers, etc. While this might not be a significant issue for light usage, it becomes cumbersome over time as users add more and more content.

One potential solution would be grouping content into "projects", as discussed in #12. However, the concept of a "project" is relatively heavy, and it would be more convenient to have something lighter to use and manage — tags. Each user content item can have multiple tags, and every list view should allow users to filter content based on tags. Each tag should include a name (required), color (required), and an optional description.