Fix width of drug selector input

I can't claim to 100% understand what was going on here, but Select2's
default sizing option was causing this input to be very slightly too
wide for the form:
https://select2.org/appearance#container-width

openprescribing/media/js/src/analyse-form.js

@@ -377,6 +377,7 @@ var queryForm = {
     };
     var optionsNum = $.extend(true, {}, select2Options);
     optionsNum.placeholder += ', e.g. Cerazette';
+    optionsNum.width = "element";
     var optionsDenom = $.extend(true, {}, select2Options);
     optionsDenom.placeholder += ', e.g. 7.3.2';
     $(this.el.numeratorIds).select2(optionsNum);