Bug Fix: Allow icons to be sized by both width and height (#6709)

## Motivation for features / changes
During the mdc-migration I restyled all the buttons via #6608 and only
recently noticed that the `mat-calendar` component (which is only used
internally) looked pretty off...

To resolve this I am allowing button widths and heights to be set
separately then overriding them in the internal date picker component
(Googlers see cl/601592566).

## Screenshots of UI changes (or N/A)

Before - https://screenshot.googleplex.com/AKuAQqzf9g6K2GF
After - https://screenshot.googleplex.com/7s6vm8SsajKg9v7

Author: rileyajones

tensorboard/webapp/theme/_tb_theme.template.scss

@@ -310,7 +310,8 @@ $tb-dark-theme: map_merge(
 
     a,
     button.mat-mdc-button-base {
-      --tb-icon-size: 24px;
+      --tb-icon-width: 24px;
+      --tb-icon-height: 24px;
       --mdc-text-button-label-text-tracking: normal;
       --mdc-filled-button-label-text-tracking: normal;
       --mdc-outlined-button-label-text-tracking: normal;
@@ -335,9 +336,9 @@ $tb-dark-theme: map_merge(
 
       mat-icon.mat-icon,
       svg {
-        width: var(--tb-icon-size);
-        height: var(--tb-icon-size);
-        line-height: var(--tb-icon-size);
+        width: var(--tb-icon-size, var(--tb-icon-width));
+        height: var(--tb-icon-size, var(--tb-icon-height));
+        line-height: var(--tb-icon-size, var(--tb-icon-height));
       }
     }
   }

tensorboard/webapp/widgets/data_table/header_cell_component.scss

@@ -55,7 +55,8 @@ $_icon_padding: 4px;
     &.context-menu-container {
       width: $_icon_size + $_icon_padding;
       height: $_icon_size + $_icon_padding;
-      --tb-icon-size: #{$_icon_size};
+      --tb-icon-width: #{$_icon_size};
+      --tb-icon-height: #{$_icon_size};
       border-radius: 5px;
       font-size: 12px;
       padding: $_icon_padding;

tensorboard/webapp/widgets/line_chart_v2/sub_view/line_chart_axis_view.scss

@@ -34,7 +34,8 @@ text {
   width: 100%;
 
   .extent-edit-button {
-    --tb-icon-size: 16px;
+    --tb-icon-width: 16px;
+    --tb-icon-height: 16px;
     height: 24px;
     position: absolute;
     right: 5px;