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/_RoomTile.scss

@@ -64,7 +64,7 @@ limitations under the License.
 
 .mx_RoomTile_subtext {
     display: inline-block;
-    font-size: 11px;
+    font-size: 0.733rem;
     padding: 0 0 0 7px;
     margin: 0;
     overflow: hidden;
@@ -112,7 +112,7 @@ limitations under the License.
 }
 
 .mx_RoomTile_name {
-    font-size: 14px;
+    font-size: 0.933rem;
     padding: 0 4px;
     color: $roomtile-name-color;
     white-space: nowrap;
@@ -126,7 +126,7 @@ limitations under the License.
     padding: 0 0.4em;
     color: $roomtile-badge-fg-color;
     font-weight: 600;
-    font-size: 12px;
+    font-size: 0.8rem;
 }
 
 .collapsed {