Rename build to pipeline for status badges
First commit in probably 2, for resolve gitlab-org/gitlab-ce#15582. This commit is renaming files and classes from build to pipeline. Also wording is editted to pipeline. Given `pipeline` had more characters than `build`, I've made the field a bit wider. The width now matchers the one for the coverage badge, so they look nice when in a table format, or in a list. As soon as this commit is merged to master, and released, the build.svg is deprecated, meaning that all users which already placed a badge should update it. However, to make sure it keeps working tests are added for this case.