Only show the first 10 lines of a spec failure and its stacktrace

It is difficult to scroll through stack traces when there are huge
failure messages. This limits the length to 10 lines, if you hover 
over the message it will expand.

Somewhat related to #1173
Remove editor class form ReactEditorView, it is on EditorComponent

I couldn’t recreate the problem with activation events mentioned
https://github.com/atom/atom/commit/fa66689c07f3c64cdf338da1c9a034d06ec54f9c