[feature:lib] Add submitted_at for submission date querying

[eonu]

Checklist

• [x] All old and new tests pass (ran bundle exec rspec spec in the root directory).
• [x] Read the contribution guidelines.
• [x] Updated documentation (if necessary).

Reason (or issue)

After reading into how it is possible to query using lastUpdatedDate in #93, it appears that it is also possible to do the same with submittedDate, so this PR implements that.

Note that instead of adding Arx::Query#submitted_date, we define it as Arx::Query#submitted_at. Arx::Query#last_updated_date is also changed to Arx::Query#updated_at. This is done in order to match the naming pattern of Arx::Paper#updated_at and Arx::Paper#published_at.

This design decision is inspired by ActiveRecord::Timestamp's created_at and updated_at.

The same changes are made to the last_updated and date_submitted sort-by criteria.