Database migration

[philhanna]

The JSON representation of grids and puzzles will change. Need to

1. Save all the existing objects in pickle format in a backup database.
2. Make the JSON changes (see issue #153)
3. Restore existing grids and puzzles from backup database:
   • Table users can just be copied from backup - no JSON
   • Likewise table words
   • For table grids:
     • Read each grid from the backup table
     • Recreate the grid from the pickled representation
     • Get new JSON output
     • Insert newly created grid into main table
   • Same for table puzzles

[philhanna]

Done.

• The script migration/create_backup.py creates a database $HOME/crossword_backup.db
• The script migration/restore_from_backup.py creates a database $HOME/crossword_new.db

The new database can be tested with a change to the dbfile entry in .crossword.ini:

[DEFAULT]
#
# dbfile -  The fully qualified path to the crossword database.
#           This is an SQLite 3 database.
# See https://www.sqlite.org/index.html
#
dbfile=/home/saspeh/crossword.db 
#dbfile=/home/saspeh/crossword_new.db 
#
# log_level -   This must one of the following:
#               CRITICAL
#               ERROR
#               WARNING
#               INFO
#               DEBUG
#               NOTSET
# See https://docs.python.org/3.7/library/logging.html#levels
#
log_level=INFO