Make all 'font-size's and 'line-height's rem

Font size of the whole app would ideally be controlled by a single
value. This value is currently hard coded using the :root CSS selector.
It is the intention to make this value configurable within riot. In the
interim all font-sizes have been converted to rem by the simple process
of regex. Replacing px values with their equivalent rem values assuming
a font size of 15px and then rounded to three decimal places, which was
the base at the time of this transformation.

I'm expecting another commit cleaning up rem values but I thought it
best to leave that to review.

This commit doesn't address any scaling issues. I thought it better to
land this unwieldy, mechanical, invisible change before the others
otherwise the pr would be impossible to review thoroughly.

res/css/views/rooms/_RoomDropTarget.scss

@@ -28,7 +28,7 @@ limitations under the License.
 }
 
 .mx_RoomDropTarget {
-    font-size: 13px;
+    font-size: 0.867rem;
     padding-top: 5px;
     padding-bottom: 5px;
     border: 1px dashed $accent-color;
@@ -41,7 +41,7 @@ limitations under the License.
 .mx_RoomDropTarget_label {
     position: relative;
     margin-top: 3px;
-    line-height: 21px;
+    line-height: 1.400rem;
     z-index: 1;
     text-align: center;
 }