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/_ImageView.scss

@@ -102,13 +102,13 @@ limitations under the License.
 }
 
 .mx_ImageView_name {
-    font-size: 18px;
+    font-size: 1.2rem;
     margin-bottom: 6px;
     word-wrap: break-word;
 }
 
 .mx_ImageView_metadata {
-    font-size: 15px;
+    font-size: 1rem;
     opacity: 0.5;
 }
 
@@ -118,13 +118,13 @@ limitations under the License.
     margin-bottom: 6px;
     border-radius: 5px;
     background-color: $lightbox-bg-color;
-    font-size: 14px;
+    font-size: 0.933rem;
     padding: 9px;
     border: 1px solid $lightbox-border-color;
 }
 
 .mx_ImageView_size {
-    font-size: 11px;
+    font-size: 0.733rem;
 }
 
 .mx_ImageView_link {
@@ -133,7 +133,7 @@ limitations under the License.
 }
 
 .mx_ImageView_button {
-    font-size: 15px;
+    font-size: 1rem;
     opacity: 0.5;
     margin-top: 18px;
     cursor: pointer;