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/structures/_MyGroups.scss

@@ -105,7 +105,7 @@ limitations under the License.
 .mx_MyGroups_placeholder {
     background-color: $info-plinth-bg-color;
     color: $info-plinth-fg-color;
-    line-height: 400px;
+    line-height: 26.667rem;
     border-radius: 10px;
     text-align: center;
 }
@@ -149,11 +149,11 @@ limitations under the License.
 
 .mx_GroupTile_profile .mx_GroupTile_name {
     margin: 0px;
-    font-size: 15px;
+    font-size: 1rem;
 }
 
 .mx_GroupTile_profile .mx_GroupTile_groupId {
-    font-size: 13px;
+    font-size: 0.867rem;
     opacity: 0.7;
 }
 
@@ -161,7 +161,7 @@ limitations under the License.
     display: -webkit-box;
     -webkit-line-clamp: 2;
     -webkit-box-orient: vertical;
-    font-size: 13px;
+    font-size: 0.867rem;
     max-height: 36px;
     overflow: hidden;
 }