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/_WhoIsTypingTile.scss

@@ -49,7 +49,7 @@ limitations under the License.
     border-radius: 40px;
     width: 24px;
     height: 24px;
-    line-height: 24px;
+    line-height: 1.600rem;
     font-size: 0.8em;
     vertical-align: top;
     text-align: center;
@@ -57,7 +57,7 @@ limitations under the License.
 
 .mx_WhoIsTypingTile_label {
     flex: 1;
-    font-size: 14px;
+    font-size: 0.933rem;
     font-weight: 600;
     color: $eventtile-meta-color;
 }