admin: reduce select height to fit form-control height

assets/scss/admin.scss

@@ -99,4 +99,10 @@ pre {
 .form-control {
     height: 30px;
     padding: 3px 12px;
+}
+.select2-container .select2-choice {
+    height: 30px;
+}
+.select2-choice>span:first-child, .select2-chosen, .select2-container .select2-choices .select2-search-field input {
+    padding: 5px 12px;
 }
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