make page views csv link text phrase more informative

Links are more useful when they make sense out of context.
We want to avoid non-informative link phrases regardless
if the context seems clear or we're under a header that
was previously read off by the screen reader.

fixes FOO-2017

test plan:
 1. Navigate to a user's profile page in Canvas
 2. Tab to csv link underneath Page Views header
 3. With VO on, verify the label now reads:
    "link, Download Page Views CSV", instead of:
    "link, csv"

Change-Id: Ife134e636fbeecac0841f63ff13a1884f68b2444
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/266318
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Michael Ziwisky <mziwisky@instructure.com>
QA-Review: Michael Ziwisky <mziwisky@instructure.com>
Product-Review: August Thornton <august@instructure.com>

app/views/page_views/_table.html.erb

@@ -33,7 +33,12 @@
         <th><%= t('headers.time', "Time") %></th>
         <th>
           <div style="float:<%= direction('right') %>; margin-<%= direction('right') %>: 4px">
-            <%= link_to("csv", user_page_views_path(@user, :format => :csv), :class => 'icon-ms-excel', :id => 'page_views_csv_link') %>
+            <%= link_to(t('page views csv'),
+                user_page_views_path(@user, :format => :csv),
+                class: 'icon-ms-excel',
+                id: 'page_views_csv_link',
+                'aria-label' => t('Download Page Views CSV'))
+            %>
           </div>
           <%= t('headers.user_agent', "User Agent") %>
         </th>