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
Merge pull request #799 from manics/rebased/develop/linux_walkthrough

Linux walkthrough (rebased onto develop)