Remove usage of underscore; replace with native JS

* Use jQuery’s `makeArray` to convert jQuery wrapped collection to native array
* Use native `Array.prototype.map` and `.some`
* Create an `isEmpty` util which is identical in behavior to underscore and lodash

app/assets/javascripts/behaviors/markdown/marks/inline_html.js

 /* eslint-disable class-methods-use-this */
 
 import { Mark } from 'tiptap';
-import _ from 'underscore';
+import { escape as esc } from 'lodash';
 
 // Transforms generated HTML back to GFM for Banzai::Filter::MarkdownFilter
 export default class InlineHTML extends Mark {
@@ -35,7 +35,7 @@ export default class InlineHTML extends Mark {
       mixable: true,
       open(state, mark) {
         return `<${mark.attrs.tag}${
-          mark.attrs.title ? ` title="${state.esc(_.escape(mark.attrs.title))}"` : ''
+          mark.attrs.title ? ` title="${state.esc(esc(mark.attrs.title))}"` : ''
         }>`;
       },
       close(state, mark) {

app/assets/javascripts/behaviors/requires_input.js

 import $ from 'jquery';
-import _ from 'underscore';
+import { isEmpty } from 'lodash';
 import '../commons/bootstrap';
 
 // Requires Input behavior
@@ -23,10 +23,10 @@ $.fn.requiresInput = function requiresInput() {
 
   function requireInput() {
     // Collect the input values of *all* required fields
-    const values = _.map($(fieldSelector, $form), field => field.value);
+    const values = Array.from($(fieldSelector, $form)).map(field => field.value);
 
     // Disable the button if any required fields are empty
-    if (values.length && _.some(values, _.isEmpty)) {
+    if (values.length && values.some(isEmpty)) {
       $button.disable();
     } else {
       $button.enable();