Add request state updating functionality to API

WIP

Adds a new `update_state` method to the API that uses the existing API key mechanism to send a status update to a request created via an external API request. Intended usage - foi-register end-users confirming whether the information they requested has been disclosed ([the parallel work on foi-register can be found here](https://github.com/mysociety/foi-register/tree/issues/169-ask-user-to-confirm-request-state)). Status description mismatches should be handled on the foi-register side but unexpected status values are rejected with a 500 error by the API.

Adds an optional parameter to the pre-existing `add_correspondence` API method to allow it to update status (intended use - updates from a public body) if a `state` value is supplied. Again, the foi-register code is responsible for handling state description mismatches but in this case the API deals with erroneous data by ignoring that part of the request.

Apologies for having restructured `api_controller_spec` but I kept losing track of which API methods the various tests were targeting. I will be happy to change it back before finalising the Pull Request if I've caused trouble.
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Subject: Re: [alaveteli] Add request state updating functionality to API
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>  
> -        request_data = {
> -            "title" => "Tell me about your chickens",
> -            "body" => "Dear Sir,\n\nI should like to know about your chickens.\n\nYours in faith,\nBob\n",
> -
> -            "external_url" => "http://www.example.gov.uk/foi/chickens_23",
> -            "external_user_name" => "Bob Smith",
> -        }
> +            request_data = {
> +                "title" => "Tell me about your chickens",
> +                "body" => "Dear Sir,\n\nI should like to know about your chickens.\n\nYours in faith,\nBob\n",
> +                "external_url" => "http://www.example.gov.uk/foi/chickens_23",
> +                "external_user_name" => "Bob Smith",

Prefer single-quoted strings when you don't need string interpolation or special symbols.

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<p>In spec/controllers/api_controller_spec.rb:</p>
<pre style='color:#555'>&gt;  
&gt; -        request_data = {
&gt; -            &quot;title&quot; =&gt; &quot;Tell me about your chickens&quot;,
&gt; -            &quot;body&quot; =&gt; &quot;Dear Sir,\n\nI should like to know about your chickens.\n\nYours in faith,\nBob\n&quot;,
&gt; -
&gt; -            &quot;external_url&quot; =&gt; &quot;http://www.example.gov.uk/foi/chickens_23&quot;,
&gt; -            &quot;external_user_name&quot; =&gt; &quot;Bob Smith&quot;,
&gt; -        }
&gt; +            request_data = {
&gt; +                &quot;title&quot; =&gt; &quot;Tell me about your chickens&quot;,
&gt; +                &quot;body&quot; =&gt; &quot;Dear Sir,\n\nI should like to know about your chickens.\n\nYours in faith,\nBob\n&quot;,
&gt; +                &quot;external_url&quot; =&gt; &quot;http://www.example.gov.uk/foi/chickens_23&quot;,
&gt; +                &quot;external_user_name&quot; =&gt; &quot;Bob Smith&quot;,
</pre>
<p>Prefer single-quoted strings when you don't need string interpolation or special symbols.</p>

<p style="font-size:small;-webkit-text-size-adjust:none;color:#666;">&mdash;<br>Reply to this email directly or <a href="https://github.com/mysociety/alaveteli/pull/1631/files#r14195066">view it on GitHub</a>.<img alt="" height="1" src="https://github.com/notifications/beacon/510285__eyJzY29wZSI6Ik5ld3NpZXM6QmVhY29uIiwiZXhwaXJlcyI6MTcxOTMzMDkyOSwiZGF0YSI6eyJpZCI6MzUzMTE5NzR9fQ==--11c9b2e30b116abde617c9883c2dde5d2914d0f1.gif" width="1" /></p>
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