Replace Node `joinPaths` with custom impl

**Why?**
The current implementation of `joinPaths` comes from the
Node core modules `path` and has a number of caveats:

- Does not support non-string values
- Does not handle joining absolute URL's

https://nodejs.org/api/path.html#path_path_join_paths

**Note:**
The new impl of `joinPaths` does not do the same special
normalization which the Node version did. This should be
safe because I'm pretty sure we only use this method to
join paths with `/` that potentially start and end with
a `/`.

app/assets/javascripts/lib/utils/url_utility.js

-import { join as joinPaths } from 'path';
+const PATH_SEPARATOR = '/';
+const PATH_SEPARATOR_LEADING_REGEX = new RegExp(`^${PATH_SEPARATOR}+`);
+const PATH_SEPARATOR_ENDING_REGEX = new RegExp(`${PATH_SEPARATOR}+$`);
 
 // Returns a decoded url parameter value
 // - Treats '+' as '%20'
@@ -6,6 +8,37 @@ function decodeUrlParameter(val) {
   return decodeURIComponent(val.replace(/\+/g, '%20'));
 }
 
+function cleanLeadingSeparator(path) {
+  return path.replace(PATH_SEPARATOR_LEADING_REGEX, '');
+}
+
+function cleanEndingSeparator(path) {
+  return path.replace(PATH_SEPARATOR_ENDING_REGEX, '');
+}
+
+/**
+ * Safely joins the given paths which might both start and end with a `/`
+ *
+ * Example:
+ * - `joinPaths('abc/', '/def') === 'abc/def'`
+ * - `joinPaths(null, 'abc/def', 'zoo) === 'abc/def/zoo'`
+ *
+ * @param  {...String} paths
+ * @returns {String}
+ */
+export function joinPaths(...paths) {
+  return paths.reduce((acc, path) => {
+    if (!path) {
+      return acc;
+    }
+    if (!acc) {
+      return path;
+    }
+
+    return [cleanEndingSeparator(acc), PATH_SEPARATOR, cleanLeadingSeparator(path)].join('');
+  }, '');
+}
+
 // Returns an array containing the value(s) of the
 // of the key passed as an argument
 export function getParameterValues(sParam, url = window.location) {
@@ -212,5 +245,3 @@ export function objectToQuery(obj) {
     .map(k => `${encodeURIComponent(k)}=${encodeURIComponent(obj[k])}`)
     .join('&');
 }
-
-export { joinPaths };

spec/frontend/lib/utils/url_utility_spec.js

@@ -298,4 +298,28 @@ describe('URL utility', () => {
       expect(urlUtils.objectToQuery(searchQueryObject)).toEqual('one=1&two=2');
     });
   });
+
+  describe('joinPaths', () => {
+    it.each`
+      paths                                       | expected
+      ${['foo', 'bar']}                           | ${'foo/bar'}
+      ${['foo/', 'bar']}                          | ${'foo/bar'}
+      ${['foo//', 'bar']}                         | ${'foo/bar'}
+      ${['abc/', '/def']}                         | ${'abc/def'}
+      ${['foo', '/bar']}                          | ${'foo/bar'}
+      ${['foo', '/bar/']}                         | ${'foo/bar/'}
+      ${['foo', '//bar/']}                        | ${'foo/bar/'}
+      ${['foo', '', '/bar']}                      | ${'foo/bar'}
+      ${['foo', '/bar', '']}                      | ${'foo/bar'}
+      ${['/', '', 'foo/bar/  ', '', '/ninja']}    | ${'/foo/bar/  /ninja'}
+      ${['', '/ninja', '/', ' ', '', 'bar', ' ']} | ${'/ninja/ /bar/ '}
+      ${['http://something/bar/', 'foo']}         | ${'http://something/bar/foo'}
+      ${['foo/bar', null, 'ninja', null]}         | ${'foo/bar/ninja'}
+      ${[null, 'abc/def', 'zoo']}                 | ${'abc/def/zoo'}
+      ${['', '', '']}                             | ${''}
+      ${['///', '/', '//']}                       | ${'/'}
+    `('joins paths $paths => $expected', ({ paths, expected }) => {
+      expect(urlUtils.joinPaths(...paths)).toBe(expected);
+    });
+  });
 });