Trim all text fields in entries before saving them in the database.

This should fix the error in feeds like https://medium.com/the-nib , which have newlines at the beginning of each entry guid. The problem is that this caused the JSON to be invalid, which meant an error appeared when the user tried to read the feed.
Added database migration to trim texts in all existing entries.

This complements commit 2afdb78083009ac753bc1c5203645e32e78aa4f9. That commit applies the fix to entries that will be saved from now on; this commit applies the fix to already existing entries.