Explicit organisations and parents

This is a first step towards fixing [transition](https://github.com/alphagov/transition)'s problems with [assigning org-specific details](https://www.pivotaltracker.com/story/show/56863446).

Without this branch, Transition is forced to infer (usually incorrect) values for organisations from site data. This is specifically a problem when an organisation has a different launch date to any of its sites, and will become more of a problem as we start to transition ALBs that are [microsites](https://github.com/alphagov/redirector/data/microsites).

This is an initial set of inferences from site YAML which has generated an organisations directory and augmented all existing sites with an organisation abbreviation (where applicable) and a parent organisation abbreviation (again, where applicable).

More work is required to fix discrepancies between organisations and sites with respect to `redirection_date`, `furl`, `homepage`.

@psd, @rjc123 - would love your comments, particularly around the [new orgs](https://github.com/alphagov/redirector/commit/fa0013cf81b0f159f4c17b983ec7d3d39195a2c4) and the augmentation of the [existing sites](https://github.com/alphagov/redirector/commit/0421c071b9b01e487f0604422a4565f47c817cf7)