Improve Jenkinsfile and GitHub status

When a branch/PR is pushed, we test a lot of stuff:

• run tests on latest Node.js version (10.x for now)
• run tests on Node.js 8.x (LTS)
• run tests on Node.js 6.x (LTS)
• run tests on Node.js 4.x with the ES5 build (to validate the ES5 build even if Node.js 4.x is EOL)
• run tests on Chrome/Firefox for the browser version of the client

For now, they are all run inside the build and test stage. The Jenkinsfile could be refactored to have:

• a lint stagge
• a build stage
• a browser build stage
• run node (latest or latest LTS?) and browser tests against Nuxeo master
• launch parallel tests on other node versions against Nuxeo master
• launch parallel tests on 7.10, 8.10 and 9.10 (Nuxeo LTS versions): those tests should be node (latest or latest LTS?) and browser tests.

Commit status could be:
lint
build/es5
build/browser
test/master/browser
test/master/node/10.x
test/master/node/8.x
test/master/node/6.x
test/master/es5
test/9.10/browser
test/9.10/node/10.x (or 8.x?)
test/8.10/browser
test/8.10/node/10.x (or 8.x?)
test/7.10/browser
test/7.10/node/10.x (or 8.x?)