Accessibility: Added labels to add feeds dialog

Fixes CNVS-13727

Test plan:
- Visit the external feeds dialog on the announcements page.
- Using a screen reader, verify that all fields are labeled.

Change-Id: I3613744e279dddaa0d99d9c9b9ddf50122b07ec4
Reviewed-on: https://gerrit.instructure.com/37090
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>

app/views/jst/ExternalFeeds/IndexView.handlebars

@@ -39,10 +39,11 @@
          id="external_feed_url"
          name="url"
          class="input-block-level"
-         placeholder="Feed URL">
+         aria-label="{{#t "feed_url_label"}}Feed URL{{/t}}"
+         placeholder="{{#t "feed_url_label"}}Feed URL{{/t}}">
   <div>
-    <select name="verbosity" class="input-block-level">
-      <option>{{#t "options.content_to_post"}}--Content to post--{{/t}}</option>
+    <select name="verbosity" class="input-block-level" aria-label="{{#t "options.content_to_post"}}Content to post{{/t}}">
+      <option>--{{#t "options.content_to_post"}}Content to post{{/t}}--</option>
       <option value="full">{{#t "options.full_article"}}Full article{{/t}}</option>
       <option value="truncate">{{#t "options.truncated"}}Truncated{{/t}}</option>
       <option value="link_only">{{#t "options.link_only"}}Link only{{/t}}</option>
@@ -59,6 +60,7 @@
            type="text"
            name="header_match"
            class="input-block-level"
+           aria-label="{{#t "phrase_to_look_for"}}Phrase to look for{{/t}}"
            placeholder="{{#t "phrase_to_look_for"}}Phrase to look for{{/t}}">
   </div>
   <div>