Semi-automatic upgrade of databases when upgrading Trac

[daniel]

Changes introduced to the Trac database in newerr versions should be easily applied to installed databases when upgrading.

Two approaches should be available:

1. Explicitly upgrade a database when requested by the user/admin.
2. Automatically upgrade if safe the database. This will only happen if the database schema is unmodified from the vanilla install.

1) is basically 'trac-admin foo.db upgrade' , but is not practical if you have several databases.
2) is performed when Trac detects an old version of a database that can be safely upgraded. It happens automatically, and the user never has to worry about upgrading. This should be default since the database should be considered embedded at all time. Trac installations using custom databases, hacked Trac code and such will not be automatically upgraded.

Implementation details

We'll have an Upgrade.py module, along with a single file per release-version containing all delta changes from the previous release. This way you can always upgrade from any older version to a later.

Automatic upgrade should be configurable as an option in the database itself.

[daniel]

Implemented in [345].