On showing diff/file, use the tab_width specified on .editorconfig, if any (#3241)

Closes #3182

public/css/gogs.css

@@ -2312,6 +2312,70 @@ footer .ui.language .menu {
 #delete-repo-modal .ui.message {
   width: 100%!important;
 }
+.tab-size-1 {
+  tab-size: 1 !important;
+  -moz-tab-size: 1 !important;
+}
+.tab-size-2 {
+  tab-size: 2 !important;
+  -moz-tab-size: 2 !important;
+}
+.tab-size-3 {
+  tab-size: 3 !important;
+  -moz-tab-size: 3 !important;
+}
+.tab-size-4 {
+  tab-size: 4 !important;
+  -moz-tab-size: 4 !important;
+}
+.tab-size-5 {
+  tab-size: 5 !important;
+  -moz-tab-size: 5 !important;
+}
+.tab-size-6 {
+  tab-size: 6 !important;
+  -moz-tab-size: 6 !important;
+}
+.tab-size-7 {
+  tab-size: 7 !important;
+  -moz-tab-size: 7 !important;
+}
+.tab-size-8 {
+  tab-size: 8 !important;
+  -moz-tab-size: 8 !important;
+}
+.tab-size-9 {
+  tab-size: 9 !important;
+  -moz-tab-size: 9 !important;
+}
+.tab-size-10 {
+  tab-size: 10 !important;
+  -moz-tab-size: 10 !important;
+}
+.tab-size-11 {
+  tab-size: 11 !important;
+  -moz-tab-size: 11 !important;
+}
+.tab-size-12 {
+  tab-size: 12 !important;
+  -moz-tab-size: 12 !important;
+}
+.tab-size-13 {
+  tab-size: 13 !important;
+  -moz-tab-size: 13 !important;
+}
+.tab-size-14 {
+  tab-size: 14 !important;
+  -moz-tab-size: 14 !important;
+}
+.tab-size-15 {
+  tab-size: 15 !important;
+  -moz-tab-size: 15 !important;
+}
+.tab-size-16 {
+  tab-size: 16 !important;
+  -moz-tab-size: 16 !important;
+}
 .organization {
   padding-top: 15px;
   padding-bottom: 80px;

public/less/_repository.less

@@ -1360,3 +1360,13 @@
 		width: 100%!important;
 	}
 }
+
+// generate .tab-size-{i} from 1 to 16
+.generate-tab-size(16);
+.generate-tab-size(@n, @i: 1) when (@i =< @n) {
+	.tab-size-@{i} {
+		tab-size: @i !important;
+		-moz-tab-size: @i !important;
+	}
+	.generate-tab-size(@n, (@i + 1));
+}