[#538] Edit install from package docs

This is some fairly heavy editing:

- Some general editing: correcting typos and formatting errors,
  re-writing some sentences in ways that (I hope) makes them clearer

- Use |solr| substitution everywhere (spells Solr consistently with a
  capital S)

- Also use |postgresql| which spells PostgreSQL with the correct caps

- Use |production.ini| substitution for path to production.ini file

- Put the note about Ubuntu 64 only right at the top, and point them to
  the source install

- Also moved _Common error messages_ link to top, where it's more
  prominent (as on source install page)

- Removed stuff about virtualbox, ec2, rackspace, etc. This seems
  unnecessary to me

- Removed "The overall process is the following:" bullet-list from the
  top, it's unnecessary and seems to imply that upgrading is part of the
  installing process which doesn't make any sense

- Numbered the major steps of the install process, like in the source
  install

- Also within major steps, broke things down into individual numbered
  steps to make things clearer and easier to follow

- Tweaked the note about installing solr and postgres on separate servers

- Rewrote the stuff at the top of the _Upgrading_ section, I hope this
  makes it a bit clearer about major vs minor versions and the two
  different upgrade processes

- Moved moving the production.ini file into its own step, so that people
  don't miss it
[#538] Don't spell SOLR in ALL-CAPS
[#538] Minor edits to package upgrade docs
Merge pull request #889 from okfn/538-package-install-docs-seanh-rewrite

[#538] Edit install from package docs