Added checkbox to control the findIfExists shlink flag

src/short-urls/UseExistingIfFoundInfoIcon.js

@@ -0,0 +1,56 @@
+import React, { useState } from 'react';
+import { FontAwesomeIcon } from '@fortawesome/react-fontawesome';
+import { faInfoCircle as infoIcon } from '@fortawesome/free-solid-svg-icons';
+import { Modal, ModalBody, ModalHeader } from 'reactstrap';
+import './UseExistingIfFoundInfoIcon.scss';
+
+const renderInfoModal = (isOpen, toggle) => (
+  <Modal isOpen={isOpen} toggle={toggle} centered size="lg">
+    <ModalHeader toggle={toggle}>Info</ModalHeader>
+    <ModalBody>
+      <p>
+        When the&nbsp;
+        <b><i>&quot;Use existing URL if found&quot;</i></b>
+        &nbsp;checkbox is checked, the server will return an existing short URL if it matches provided params.
+      </p>
+      <p>
+        These are the checks performed by Shlink in order to determine if an existing short URL should be returned:
+      </p>
+      <ul>
+        <li>
+          When only the long URL is provided: The most recent match will be returned, or a new short URL will be created
+          if none is found
+        </li>
+        <li>
+          When long URL and custom slug are provided: Same as in previous case, but it will try to match the short URL
+          using both the long URL and the slug.
+          <br />
+          If the slug is being used by another long URL, an error will be returned.
+        </li>
+        <li>
+          When other params are provided: Same as in previous cases, but it will try to match existing short URLs with
+          all provided data. If any of them does not match, a new short URL will be created
+        </li>
+      </ul>
+      <blockquote className="use-existing-if-found-info-icon__modal-quote">
+        <b>Important:</b> This feature will be ignored while using a Shlink version older than v1.16.0.
+      </blockquote>
+    </ModalBody>
+  </Modal>
+);
+
+const UseExistingIfFoundInfoIcon = () => {
+  const [ isModalOpen, setIsModalOpen ] = useState(false);
+  const toggleModal = () => setIsModalOpen(!isModalOpen);
+
+  return (
+    <React.Fragment>
+      <span title="What does this mean?">
+        <FontAwesomeIcon icon={infoIcon} style={{ cursor: 'pointer' }} onClick={toggleModal} />
+      </span>
+      {renderInfoModal(isModalOpen, toggleModal)}
+    </React.Fragment>
+  );
+};
+
+export default UseExistingIfFoundInfoIcon;