Add Linux walkthrough scripts.

This is a direct copy of the files in
https://github.com/manics/omero-install/commit/6ca41e4d77f57eaad155a4686ea68c5325a7daca
Add Linux walkthrough document referencing the walkthrough scripts
Fix typos
Use American spelling/grammar
Fix download file titles
Fix postgres startup, use full path to omero
Add note about settings.env, clarify nginx setup
Update to 5.0.2 release URL
Remove unneeded cd ~omero, add comment about commented lines
Make some system usernames bold
Add documentation about the OMERO upgrade CI jobs
Add table summarizing the deployment servers for the CI jobs
Remove unsupported Ice 3.3 from the 5.1.x series
Clarify trigger jobs workflow
Fix erroneous copy and paste
Add MIP docs to webgateway page
Update DropBox documentation

A minor change to make a couple of sentences more grammatical.
Merge pull request #816 from will-moore/rebased/develop/webgateway_mip_5150

Webgateway mip 5150 (rebased onto develop)
Fix description of upgrade jobs
Merge pull request #812 from sbesson/50_job_renaming2

Remaining changes post 5.0.x job migration