Create new users to circumvent the GitHub API rate limit

We are already reaching the GitHub API rate limit that is 5000 calls/hour, we must understand why (and try to fix it).

• audit all the Github API calls we are doing;
• limit our use of GitHub status on multibranch projects?
• limit multibranch project to build only PRs? For instance, that job https://qa.nuxeo.org/jenkins/job/master/job/nuxeo-master-sonar/ is probably making one call when checking each PR/branch, ~120 calls as there are more than 100 branches (see https://qa.nuxeo.org/jenkins/job/master/job/nuxeo-master-sonar/indexing/consoleText). The indexing is done each time there is a push on the nuxeo repository...