CI: install yarn like TerriaJS

[pjonsson]

This copies the more recent CI-setup
from TerriaJS, which eliminates
the need to keep a copy of an old
version of yarn in the repository.

This eliminates a lot of warnings
when doing a project scan for problems
in WebStorm.

Edit: problems detected by WebStorm with default settings before/after: 629 errors, 4819 warnings, 14459 weak warnings vs 628 errors, 593 warnings, 1018 weak warnings. The scan time also goes from taking minutes on my desktop machine to passing in a few seconds.

[pjonsson]

This repository never seems to run the CI until one of you approve it. Can the CI settings be changed so I can make the PR green before you have to look at it?

[zoran995]

IMO it is not a good idea to remove those files from the repo as those enforce the yarn version that works properly with terriajs codebase. We would first need to support the latest version of yarn before removing those files.

[pjonsson]

This PR mirrors how it is already done in the terriajs repository. (And we're in trouble if any terria-packages will only work if they are installed with an ancient yarn that nobody uses.)