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/messages/_MessageActionBar.scss

@@ -21,7 +21,7 @@ limitations under the License.
     cursor: pointer;
     display: flex;
     height: 24px;
-    line-height: 24px;
+    line-height: 1.600rem;
     border-radius: 4px;
     background: $message-action-bar-bg-color;
     top: -18px;