eslint: Enable space-infix-ops rule.

More about rule at  https://eslint.org/docs/rules/space-infix-ops

static/js/compose_state.js

@@ -30,7 +30,7 @@ function get_or_set(fieldname, keep_leading_whitespace) {
     // because the DOM element might not exist yet when get_or_set
     // is called.
     return function (newval) {
-        var elem = $('#'+fieldname);
+        var elem = $('#' + fieldname);
         var oldval = elem.val();
         if (newval !== undefined) {
             elem.val(newval);