add deployment to docs, update install pages accordingly

Also added glossary entries for dev/staging/production.
The deployment text is from doc/DEPLOY.md which will can be removed
if this goes live into code.alaveteli.org
add setting STAGING_SITE to dev/staging/production defs

this sets the production environment in Rails, nice to have this documented now :-)
tidy up wording
clarifications to deploy page
fix title error (Installing, not Deploying!)
insert missing word
copyedit: remove unclear 'However'
explicit link to installation from deploy page
add Capistrano to the glossary
fixes to comments/input from @crowbot

This includes the deploy/Capistrano instructions from #1596
Merge branch 'gh-pages' into gh-pages-deploy-docs
Merge branch 'gh-pages-deploy-docs' into gh-pages