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/_RoomDirectory.scss

@@ -64,7 +64,7 @@ limitations under the License.
 }
 
 .mx_RoomDirectory_table {
-    font-size: 12px;
+    font-size: 0.8rem;
     color: $primary-fg-color;
     width: 100%;
     text-align: left;
@@ -112,7 +112,7 @@ limitations under the License.
 
 .mx_RoomDirectory_name {
     display: inline-block;
-    font-size: 18px;
+    font-size: 1.2rem;
     font-weight: 600;
 }
 
@@ -124,7 +124,7 @@ limitations under the License.
     border-radius: 10px;
     display: inline-block;
     height: 20px;
-    line-height: 20px;
+    line-height: 1.333rem;
     padding: 0 5px;
     color: $accent-fg-color;
     background-color: $rte-room-pill-color;
@@ -136,7 +136,7 @@ limitations under the License.
 }
 
 .mx_RoomDirectory_alias {
-    font-size: 12px;
+    font-size: 0.8rem;
     color: $settings-grey-fg-color;
 }
 
@@ -150,7 +150,7 @@ limitations under the License.
 }
 
 .mx_RoomDirectory > span {
-    font-size: 15px;
+    font-size: 1rem;
     margin-top: 0;
 
     .mx_AccessibleButton {