Reconsider repository structure

I find it odd that:

1. The structure encourages users to "create their own version" of a scalar if they disagree with something.

Shouldn't the official GraphQL Scalars repository encourage the opposite – a single canonical definition of each scalar, that all or most implementers agree on?

How will we benefit from `glen/date-time`, `andreas/date-time`, `martin/date-time`, etc.? And why do I need this repository if I could just host my spec under a personal or business URL?

2. Contributor names are included in the spec URLs.

IMO, the URLs should have the format:

`https://scalars.graphql.org/date-time`

This could be extended with a version number when changes are made:

`https://scalars.graphql.org/date-time/v2`

It feels off including usernames in spec URLs.

Related: #34.