Make max JSON request size configurable and default to 8 MB

This is to allow multi-document updates, for example when creating a new
project from a zip file.

services/document-updater/test/acceptance/js/SettingADocumentTests.js

@@ -230,8 +230,7 @@ describe('Setting a document', function () {
       })
       this.newLines = []
       while (
-        JSON.stringify(this.newLines).length <
-        Settings.max_doc_length + 64 * 1024
+        JSON.stringify(this.newLines).length <= Settings.maxJsonRequestSize
       ) {
         this.newLines.push('(a long line of text)'.repeat(10000))
       }