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/_Field.scss

@@ -40,7 +40,7 @@ limitations under the License.
 .mx_Field textarea {
     font-weight: normal;
     font-family: $font-family;
-    font-size: 14px;
+    font-size: 0.933rem;
     border: none;
     // Even without a border here, we still need this avoid overlapping the rounded
     // corners on the field above.
@@ -102,7 +102,7 @@ limitations under the License.
         background-color 0.25s ease-out 0.1s;
     color: $primary-fg-color;
     background-color: transparent;
-    font-size: 14px;
+    font-size: 0.933rem;
     position: absolute;
     left: 0px;
     top: 0px;
@@ -126,7 +126,7 @@ limitations under the License.
         color 0.25s ease-out 0s,
         top 0.25s ease-out 0s,
         background-color 0.25s ease-out 0s;
-    font-size: 10px;
+    font-size: 0.667rem;
     top: -13px;
     padding: 0 2px;
     background-color: $field-focused-label-bg-color;