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
Edit on GitHub extension for sphinx
Fix edit_on_github extension
Move _ext and _templates to common/
Add .gitignore to ext/
Add option to specify edit_on_github_prefix value

Fix omero/conf.py to use 'omero' as the prefix
Fix Edit on GitHub branch and prefixes
Update index.txt

Add sentence about the new "Edit on GitHub" button
as a test of the new "Edit on GitHub" button.

Cheers,
~@joshmoore
Add missing prefixes
Merge pull request #801 from sbesson/rebased/develop/edit-button

Add Edit on Github button (rebased onto develop)
Merge pull request #802 from snoopycrimecop/patch-1

Update index.txt