Move schema changes to the contributing documentation

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/12072

This PR primarily moves the  schema changes  section from the OMERO developer documentation to the Contributing documentation. Additionally, 
- the `commit` extlink is moved to the `contributing` section exclusively
- the description of the breaking upgrade job is improved
- the schema page is updated to add references to the breaking jobs
- the SQL upgrade scripts are now necessary both from the last release and the last dbpatch version

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--no-rebase