Change primary branch to "8.x-1.x"

Problem/Motivation

The current branch has the name "@8.x-1.x". This doesn't match Drupal's branch naming convention so might lead to unexpected problems, and at the very least makes it harder to download for people unfamiliar with the command line.

Proposed resolution

Create a new branch called "8.x-1.x" from the current "@8.x-1.x" branch.

Remaining tasks

Create the new branch.

User interface changes

n/a

API changes

n/a

Data model changes

n/a