Languages now refer to other languages they are translated to through
an additional table rather than an array of identifiers. This ensures
no orphan identifier remains when a language is deleted.
The primary key of languages is now a unique identifier rather than
the name of the language itself. It now allows for multiple languages
to have the same name. Their unique identifier is now a v4 UUID.
Set Diesel to specific version 2.0.2, since 2.0 apparently does not
mean the latest version of 2.0.z and 2.0 has issues with its uuid
feature.
Cleanup and simplify some code.
Some more GraphQL documentation on available queries.
