Clean up Bundler guidelines

* Remove old Bundler recommendation in Rails section, which has been
  superceded by the Bundler section.
* Alphabetize Bundler guidelines.
* Wrap at 80 characters where possible.
* Add binstubs guideline.
Add database index guidelines

Extracted from old thoughtbot development handbook.
Add Jasmine guideline

Extracted and simplified from old thoughtbot development handbook.
Add HTML and CSS best practices

Extracted from old thoughtbot development handbook.
Remove "Avoid absolute positioning" guideline