Provide an <input type="file"> control in Remarkup for mobile and users with esoteric windowing systems

Summary:
Ref T5187. This definitely feels a bit flimsy and I'm going to hold it until I cut the release since it changes a couple of things about Workflow in general, but it seems to work OK and most of it is fine.

The intent is described in T5187#176236.

In practice, most of that works like I describe, then the `phui-file-upload` behavior gets some weird glue to figure out if the input is part of the form. Not the most elegant system, but I think it'll hold until we come up with many reasons to write a lot more Javascript.

Test Plan:
Used both drag-and-drop and the upload dialog to upload files in Safari, Firefox and Chrome.

{F1653716}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5187

Differential Revision: https://secure.phabricator.com/D15953

webroot/rsrc/js/phui/behavior-phui-file-upload.js

@@ -0,0 +1,80 @@
+/**
+ * @provides javelin-behavior-phui-file-upload
+ * @requires javelin-behavior
+ *           javelin-stratcom
+ *           javelin-dom
+ *           phuix-dropdown-menu
+ */
+
+JX.behavior('phui-file-upload', function(config) {
+
+  function startUpload(workflow, input) {
+    var files = input.files;
+
+    if (!files || !files.length) {
+      return;
+    }
+
+    var state = {
+      workflow: workflow,
+      input: input,
+      waiting: 0,
+      phids: []
+    };
+
+    var callback = JX.bind(null, didUpload, state);
+
+    var dummy = input;
+    var uploader = new JX.PhabricatorDragAndDropFileUpload(dummy)
+      .setURI(config.uploadURI)
+      .setChunkThreshold(config.chunkThreshold);
+
+    uploader.listen('didUpload', callback);
+    uploader.start();
+
+    workflow.pause();
+    for (var ii = 0; ii < files.length; ii++) {
+      state.waiting++;
+      uploader.sendRequest(files[ii]);
+    }
+  }
+
+  function didUpload(state, file) {
+    state.phids.push(file.getPHID());
+    state.waiting--;
+
+    if (state.waiting) {
+      return;
+    }
+
+    state.workflow
+      .addData(config.inputName, state.phids.join(', '))
+      .resume();
+  }
+
+  JX.Workflow.listen('start', function(workflow) {
+    var form = workflow.getSourceForm();
+    if (!form) {
+      return;
+    }
+
+    var input;
+    try {
+      input = JX.$(config.fileInputID);
+    } catch (ex) {
+      return;
+    }
+
+    var local_form = JX.DOM.findAbove(input, 'form');
+    if (!local_form) {
+      return;
+    }
+
+    if (local_form !== form) {
+      return;
+    }
+
+    startUpload(workflow, input);
+  });
+
+});