Data consistency protection

[Daandelange]

Currently, the config's tracking period must match the database's one.
Database files aren't exchangeable with different tracking period configurations.
Corruption cases: when the configuration suddenly changes or when databases are manually interchanged with a period config mismatch.

To protect the database data :

• Prevent writing to it when the configuration's tracking period doesn't match the one from the db.
  Make it backup the old one and create a new one ? And enforce period name in db filename + db contents ?
  Also discussed in Clarify TimeFrame switching and notify on user mistakes #22
• Prevent overwriting existing incompatible db files by renaming them ?
• Add an option to explicitly disable generating stats. ( see: daandelange.simplestats.tracking.method)
• Protect the database when adding/removing Kirby languages and write an upgrade script so data can be kept.