Add search.contextLines to set number of context lines in search results

Created by: attfarhan

Partially addresses https://github.com/sourcegraph/sourcegraph/issues/2432 by adding a setting field to set the number default number of context lines displayed in search results.

I named this "search.defaultContextLines" since this sets the default for the user or instance, whereas the issue specifically asks for a search token which configures this at query time, which I foresee us adding soon. It's called search.contextLines