Allow selection of headlines

Today it's not possible to select headlines with your mouse so that you can copy it. This is important for the APM Node.js agent as the headlines in many cases are command that the user needs to type, so not being able to select and copy them is really annoying.

Here's an example of one such headline:

https://www.elastic.co/guide/en/apm/agent/nodejs/current/agent-api.html#error-on-aborted-requests