Implement code coverage badge and some other improvements regarding code coverage

We use the test coverage parsing feature to display the coverage percentage on the build page. Works like a charm, but please consider the following suggestions:

• Add a code coverage badge (like the Gitlab CI build badge): We don't want to use an external service like coveralls, especially since the information is already available in Gitlab;

• Display the code coverage percentage - if available - on the builds and commits page. It is valuable information. Now I have to click into the build page or go the the build section on the commit page.

• A global default setting for the Test coverage parsing regex setting. We now have to set this setting for each new project. It would be great if we could set the default value in the global Gitlab settings.

Thanks for all the hard work! Gitlab rocks!