Allow typing a toot while an image uploads (#1429)

This patch stops disabling the toot text field when an image is uploading. Instead, you can type to your heart's content and when the image uploads it'll append the image URL, and restore the position of your cursor to wherever it was in the toot text just prior. Effectively, the image URL is appended to the toot, and typing is not interrupted at all! ✨

app/assets/javascripts/components/features/compose/components/compose_form.jsx

@@ -83,11 +83,23 @@ const ComposeForm = React.createClass({
     this.props.onChangeSpoilerText(e.target.value);
   },
 
+  componentWillReceiveProps (nextProps) {
+    // If this is the update where we've finished uploading,
+    // save the last caret position so we can restore it below!
+    if (!nextProps.is_uploading && this.props.is_uploading) {
+      this._restoreCaret = this.autosuggestTextarea.textarea.selectionStart;
+    }
+  },
+
   componentDidUpdate (prevProps) {
-    if (this.props.focusDate !== prevProps.focusDate) {
-      // If replying to zero or one users, places the cursor at the end of the textbox.
-      // If replying to more than one user, selects any usernames past the first;
-      // this provides a convenient shortcut to drop everyone else from the conversation.
+    // This statement does several things: 
+    // - If we're beginning a reply, and,
+    //     - Replying to zero or one users, places the cursor at the end of the textbox.
+    //     - Replying to more than one user, selects any usernames past the first;
+    //       this provides a convenient shortcut to drop everyone else from the conversation.
+    // - If we've just finished uploading an image, and have a saved caret position,
+    //   restores the cursor to that position after the text changes!
+    if (this.props.focusDate !== prevProps.focusDate || (prevProps.is_uploading && !this.props.is_uploading && typeof this._restoreCaret === 'number')) {
       let selectionEnd, selectionStart;
 
       if (this.props.preselectDate !== prevProps.preselectDate) {
@@ -118,7 +130,7 @@ const ComposeForm = React.createClass({
 
   render () {
     const { intl, needsPrivacyWarning, mentionedDomains, onPaste } = this.props;
-    const disabled = this.props.is_submitting || this.props.is_uploading;
+    const disabled = this.props.is_submitting;
 
     let publishText    = '';
     let privacyWarning = '';