Manage lock files more smoothly

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Prerequisites

• I have read the Contributing guidelines
• I have read the Code of conduct and I agree with it

Description

We are currently saving both npm 5 and yarn lock files in github.
For sure it is not realistic to ask of contributors to think of updating both these files when adding or upgrading dependencies.
There is currently a discussion at yarnpkg/yarn#3614 about supporting npm 5 package-lock.json, which would be nice...

In the mean time, it seems there is no elegant way of dealing with this.
We could choose to keep only one of the lock files, but we can not make sure that contributors will use npm over yarn or yarn over npm.

Currently npm seems to suffer a minor drawback in comparison to yarn, related to unwanted updates to package-lock.json when switching between Linux (or Windows !) and Mac npm/npm#17722, in particular with fsevents which is a Mac specific dependency.
@frinyvonnick Have you used npm 5 and experienced unwanted package-lock.json updates on the project ?

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Things are not moving very fast at npm, so for now we'll only support yarn.
package-lock.json will be removed and ignored.

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In addition we'll probably use yarn workspaces with lerna