Text responses tab: add textResponsesRedux

[maddiedierker]

In order to convert the "text responses" tab in teacher dashboard to react, we will want student text response data to be available in redux. This PR creates the reducer with a way to pull responses into redux from dashboardapi and controls loading states since the former action is asynchronous.

Since a teacher will always be viewing text responses in the scope of a section and a script, theresponseData object in redux state looks as follows (some response properties removed for clarity):

responseData = {
  scriptId: [{
    question: "Who's your favorite programmer?",
    response: "Grace Hopper!",
    studentName: "Maddie",
    studentId: 1
  }]
}

responseData is then cleared when the teacher switches sections. This structure 1) allows the reducer to keep track of which scripts it has already loaded and will not re-request that data from the server if it already has it, and 2) makes rendering the response table more straightforward.

[caleybrock]

Looks good to me!

[caleybrock]

nit: this is pretty long - should be broken up into two.

[caleybrock]

could be useful to have a comment explaining that responseData is an object where the key is a script, and the value is etc.

In progress, we appended "byScript" to our names to make it a bit more clear: https://github.com/code-dot-org/code-dot-org/blob/staging/apps/src/templates/sectionProgress/sectionProgressRedux.js#L153