Refactor cookbook partial SCSS to be more generic

Since the styles for the tools partial are the same as the
cookbook partial this refactors the styles for the cookbook partial so
the class names are more generic so they may be used for tools.
Add tools index

Add a tools index that lists all tools.
Change Tool#user to Tool#owner

For consistency with Cookbook this changes Tool#user to Tool#owner and
adds a convenience method Tool#maintainer that fetches the owners
username.
Support tools atom feed
Remove useless cookbook classes.
Support filtering tools by type

Allow tools to be filtered by type based on Tool::ALLOWED_TYPES.
Limit allowed tool types

Limit and validate the allowed type of tools that can be created.