translate "File Storage" course statistics labels

fixes FOO-2290
flag = none

the phrases do have a translation, only that the before_label() helper
expects a key to be provided that wasn't:

    before_label("foo", "Default english translation")
    # => { key: "labels.foo", default: "Default english translation" }

::::::::::::;
::test plan::
:::::::::::::

- switch to some non-english locale
- visit /courses/:id/statistics
- navigate to teh "File Storage" tab (last one)
- verify the labels for "Allotted Storage", "Uploaded files" and "Media
  files" are all translated

Change-Id: Ieba41c1c7dcb33fe05916942df67d45ef4f91b32
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/273031
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: August Thornton <august@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>

app/views/courses/statistics.html.erb

@@ -186,19 +186,19 @@
       <h2><%= t('File Storage') %></h2>
       <table>
         <tr>
-          <td><%= before_label('Allotted Storage') %></td>
+          <td><%= before_label('allotted_storage', 'Allotted Storage') %></td>
           <td style="padding-<%= direction('left') %>: 10px; text-align: <%= direction('right') %>;"><%= number_to_human_size(@context.quota) %></td>
           <td></td>
         </tr>
 
         <tr>
-          <td><%= before_label('Uploaded Files') %></td>
+          <td><%= before_label('uploaded_files', 'Uploaded Files') %></td>
           <td style="padding-<%= direction('left') %>: 10px; text-align: <%= direction('right') %>;"><%= number_to_human_size(@files_size) %></td>
           <td style="padding-<%= direction('left') %>: 10px; text-align: <%= direction('right') %>;"><%= t 'file_count', 'file', :count => @file_count %></td>
         </tr>
 
         <tr>
-          <td><%= before_label('Media Files') %></td>
+          <td><%= before_label('media_files', 'Media Files') %></td>
           <td style="padding-<%= direction('left') %>: 10px; text-align: <%= direction('right') %>;"><%= number_to_human_size(@media_files_size) %></td>
           <td style="padding-<%= direction('left') %>: 10px; text-align: <%= direction('right') %>;"><%= t 'file_count', 'file', :count => @media_file_count %></td>
         </tr>