Migrate less from v2 to v4

v2 is no longer maintained.

The only incompatibility I found that affects us is that divisions must
now be inside parentheses.

Author: mwiencek

package.json

@@ -43,7 +43,7 @@
     "knockout-arraytransforms": "git://github.com/mwiencek/knockout-arraytransforms.git#9673e91",
     "leaflet": "1.6.0",
     "leaflet.markercluster": "1.4.1",
-    "less": "2.7.3",
+    "less": "4.1.1",
     "less-loader": "4.1.0",
     "less-plugin-clean-css": "1.5.1",
     "leven": "2.0.0",

root/static/styles/forms.less

@@ -250,7 +250,7 @@ table.row-form {
     > tbody > tr, > thead > tr {
         > td:first-child, > th:first-child {
             min-width: @form-label-width;
-            padding: @form-margin / 2;
+            padding: (@form-margin / 2);
             padding-bottom: 9px;
             box-sizing: border-box;
             vertical-align: top;
@@ -619,7 +619,7 @@ table.artist-credit-editor {
 #external-links-editor {
     td {
         vertical-align: top;
-        padding-top: @form-margin / 2;
+        padding-top: (@form-margin / 2);
 
         &:first-child {
             min-width: 75px;
@@ -633,7 +633,7 @@ table.artist-credit-editor {
 
     .external-link-item {
         td {
-            padding-bottom: @form-margin / 4;
+            padding-bottom: (@form-margin / 4);
         }
 
         td:first-child {
@@ -660,15 +660,15 @@ table.artist-credit-editor {
 
     .relationship-item {
         td {
-            padding-top: @form-margin / 4;
-            padding-bottom: @form-margin / 4;
+            padding-top: (@form-margin / 4);
+            padding-bottom: (@form-margin / 4);
         }
 
         div.relationship-content {
             display: flex;
 
             label {
-                margin-right: @form-margin / 2;
+                margin-right: (@form-margin / 2);
                 white-space: nowrap;
             }
 
@@ -700,7 +700,7 @@ table.artist-credit-editor {
 
 #external-links-editor, #work-attributes {
     width: 100%;
-    padding: @form-margin / 2;
+    padding: (@form-margin / 2);
 
     td:first-child select {
         max-width: @form-label-width - @form-margin;
@@ -782,7 +782,7 @@ div.rel-editor-dialog {
     .change-direction { padding: 2px !important; }
 
     .attribute-container {
-        margin-bottom: @form-margin / 2;
+        margin-bottom: (@form-margin / 2);
     }
 
     .attribute-credit {