fix: personal server invite link omitted base URL (#2900)

* fix: personal server invite link omitted base URL

* fix: respect `baseURL` when building invite URL

* test: add simple unit test for <RegistrationLink>

* clean up test

Co-authored-by: Scott Bishel <scott.bishel@mattermost.com>

webapp/src/components/sidebar/__snapshots__/registrationLink.test.tsx.snap

@@ -0,0 +1,65 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`components/sidebar/RegistrationLink renders with signupToken in URL query param 1`] = `
+<div>
+  <div
+    class="Modal bottom-right"
+  >
+    <div
+      class="toolbar hideOnWidescreen"
+    >
+      <button
+        aria-label="Close"
+        class="IconButton"
+        title="Close"
+        type="button"
+      >
+        <i
+          class="CompassIcon icon-close CloseIcon"
+        />
+      </button>
+    </div>
+    <div
+      class="RegistrationLink"
+    >
+      <div
+        class="row"
+      >
+        Share this link for others to create accounts:
+      </div>
+      <div
+        class="row"
+      >
+        <a
+          class="shareUrl"
+          href="http://localhost/register?t=abc123"
+          rel="noreferrer"
+          target="_blank"
+        >
+          http://localhost/register?t=abc123
+        </a>
+        <button
+          class="Button filled size--small"
+          type="button"
+        >
+          <span>
+            Copy link
+          </span>
+        </button>
+      </div>
+      <div
+        class="row"
+      >
+        <button
+          class="Button emphasis--secondary size--small"
+          type="button"
+        >
+          <span>
+            Regenerate token
+          </span>
+        </button>
+      </div>
+    </div>
+  </div>
+</div>
+`;

webapp/src/components/sidebar/registrationLink.test.tsx

@@ -0,0 +1,43 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import React from 'react'
+import configureStore from 'redux-mock-store'
+
+import {Provider as ReduxProvider} from 'react-redux'
+
+import {render} from '@testing-library/react'
+
+import {wrapIntl} from '../../testUtils'
+
+import RegistrationLink from './registrationLink'
+
+describe('components/sidebar/RegistrationLink', () => {
+    const mockStore = configureStore([])
+    const state = {
+        teams: {
+            current: {
+                id: 'team-id',
+                signupToken: 'abc123'
+            },
+        },
+    }
+
+    test('renders with signupToken in URL query param', () => {
+        const store = mockStore(state)
+
+        const component = wrapIntl(
+            <ReduxProvider store={store}>
+                <RegistrationLink
+                    onClose={() => {}}
+                />
+            </ReduxProvider>,
+        )
+        const {container} = render(component)
+        expect(container).toMatchSnapshot()
+
+        const anchor = container.querySelector('.shareUrl')
+        const url = new URL(anchor?.getAttribute('href') as string)
+        expect(url.searchParams.get('t')).toStrictEqual(state.teams.current.signupToken)
+    })
+})

webapp/src/components/sidebar/registrationLink.tsx

@@ -44,7 +44,7 @@ const RegistrationLink = (props: Props) => {
         }
     }
 
-    const registrationUrl = Utils.buildURL('/register?t=' + signupToken, true)
+    const registrationUrl = `${Utils.getBaseURL(true).replace(/\/$/, '')}/register?t=${signupToken}`
 
     return (
         <Modal