WIP: automatically delete old entries, so that database size does not grow indefinitely.

Currently implemented: created DeletedEntry model; the idea is that when entries are deleted in an automated cleanup, a DeletedEntry instance is saved in the db. This will be used in the future to ensure that an already deleted entry is not saved again in the database.

This means that deleted entries will occupy some space in the database, and database size will grow over time. However each deleted entry will need very little database space (specially because the entry content, which is by far the biggest field, won't be kept) and the growth should be manageable. Also, the read/unread states for deleted entries won't be kept, which also means some space saving.
Added log when an entry cannot be saved because it was deleted in the past.
Added unit tests for DeletedEntry model.
Automatically delete old entries.

Every time a scheduled feed update is run, after inserting any new entries, the number of entries in the feed is counted to check if it is above the limit configured in application.rb. If it is, older entries are deleted until the number of entries in the feed is the maximum allowed.

By "delete entries" I mean that the Entry record is deleted in the database, and a corresponding DeletedEntry record is inserted in the db so that the entry will not be saved again in the database, even if the feed server returns it again in the future. Destroying Entry instances also triggers the destruction of associated EntryStates.

This should keep the database from growing too fast, keeping old entries indefinitely does not add much value.