OMERO configuration properties (rebased onto develop)


This is the same as gh-900 but rebased onto develop.

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This PR :
- reviews the source code links to `et/comero.properties` across the documentation
- replaces them by links to the `sysadmins/config` page whenever appropriate
- review all references to the server configuration properties and uses the `:term:` markup when possible

                
Should `omero.upgrades.url` be a `:term:`?