Refactor SummaryText

As @sarathms wrote in: https://github.com/ooni/explorer/pull/146/#discussion_r282578340

Turns out it is a bug, yet works as expected. It was certainly not intended this way. react-intl fails to find the key and prints the key itself as the message.

I have modified SummaryText to accept either a function, string, or any in the content prop and render them accordingly. The problem with string type is that it cannot distinguish between the react-intl key and an actual string to be rendered (in this case already formatted).

One fix is sending the key as an object to the content prop e.g content={{ key: 'Measurement.Test.SummaryText.Reason'}} with additional values if needed. And handle it the way typeof content === 'string' is handled right now. And render strings as is.

I feel the measurement details section in general and the SummaryText component in particular have gone through many iterations and I haven't been able to apply the patterns consistently across components/measurement/nettests/*.js files. I wonder if I should refactor it based on what we know now about what we want.