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/dialogs/_SetMxIdDialog.scss

@@ -29,7 +29,7 @@ limitations under the License.
     padding: 9px;
     color: $primary-fg-color;
     background-color: $primary-bg-color;
-    font-size: 15px;
+    font-size: 1rem;
     width: 100%;
     max-width: 280px;
 }