This adds a new "nextversion" field to `question_bank_entries` to track
which version number the next version of the question should have. This
ensures that version numbers are not re-used if the latest version is
deleted.
To use this field, you must call
`\core_question\versions::get_next_version()`. This will initialise the
field to the correct value if it is currently null.
If you create a new version using this value, you must then call
`\core_question\versions::increment_next_version()` to increment the
counter.
Mark Johnson