Use variables for the rem values.

It's become obvious that these random floating points everywhere
are unwieldy. Now they're all in one place with some fairly logical
variable names which will help out in design->implementation phase.

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: 0.933rem;
+    font-size: $font-14px;
     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: 0.933rem;
+    font-size: $font-14px;
     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: 0.667rem;
+    font-size: $font-10px;
     top: -13px;
     padding: 0 2px;
     background-color: $field-focused-label-bg-color;