Update Linux walkthrough to 5.0.5 (rebased onto develop)

This is the same as gh-998 but rebased onto develop.

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Note Nginx installation has been rearrranged to reduce duplication when writing scripts for an Apache install (not included in the walkthrough, see https://github.com/manics/omero-install if you're interested).

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Also note that the server version is kept as 5.0.5 to avoid desynchronising the docs/scripts.