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/elements/_EventListSummary.scss

@@ -19,7 +19,7 @@ limitations under the License.
 }
 
 .mx_TextualEvent.mx_EventListSummary_summary {
-    font-size: 14px;
+    font-size: 0.933rem;
     display: inline-flex;
 }
 
@@ -27,7 +27,7 @@ limitations under the License.
     display: inline-block;
     margin-right: 8px;
     padding-top: 8px;
-    line-height: 12px;
+    line-height: 0.8rem;
 }
 
 .mx_EventListSummary_avatars .mx_BaseAvatar {
@@ -46,19 +46,19 @@ limitations under the License.
 .mx_EventListSummary_line {
     border-bottom: 1px solid $primary-hairline-color;
     margin-left: 63px;
-    line-height: 30px;
+    line-height: 2.000rem;
 }
 
 .mx_MatrixChat_useCompactLayout {
     .mx_EventListSummary {
-        font-size: 13px;
+        font-size: 0.867rem;
         .mx_EventTile_line {
-            line-height: 20px;
+            line-height: 1.333rem;
         }
     }
 
     .mx_EventListSummary_line {
-        line-height: 22px;
+        line-height: 1.467rem;
     }
 
     .mx_EventListSummary_toggle {
@@ -66,6 +66,6 @@ limitations under the License.
     }
 
     .mx_TextualEvent.mx_EventListSummary_summary {
-        font-size: 13px;
+        font-size: 0.867rem;
     }
 }