Fix Web IDE terminal button on relative URL root

app/assets/javascripts/ide/services/terminals.js

 import axios from '~/lib/utils/axios_utils';
+import { joinPaths } from '~/lib/utils/url_utility';
 
-export const baseUrl = (projectPath) => `/${projectPath}/ide_terminals`;
+export const baseUrl = (projectPath) =>
+  joinPaths(gon.relative_url_root || '', `/${projectPath}/ide_terminals`);
 
 export const checkConfig = (projectPath, branch) =>
   axios.post(`${baseUrl(projectPath)}/check_config`, {

spec/frontend/ide/services/terminals_spec.js

+import MockAdapter from 'axios-mock-adapter';
+import * as terminalService from '~/ide/services/terminals';
+import axios from '~/lib/utils/axios_utils';
+
+const TEST_PROJECT_PATH = 'lorem/ipsum/dolar';
+const TEST_BRANCH = 'ref';
+
+describe('~/ide/services/terminals', () => {
+  let axiosSpy;
+  let mock;
+  const prevRelativeUrlRoot = gon.relative_url_root;
+
+  beforeEach(() => {
+    axiosSpy = jest.fn().mockReturnValue([200, {}]);
+
+    mock = new MockAdapter(axios);
+    mock.onPost(/.*/).reply((...args) => axiosSpy(...args));
+  });
+
+  afterEach(() => {
+    gon.relative_url_root = prevRelativeUrlRoot;
+    mock.restore();
+  });
+
+  it.each`
+    method           | relativeUrlRoot | url
+    ${'checkConfig'} | ${''}           | ${`/${TEST_PROJECT_PATH}/ide_terminals/check_config`}
+    ${'checkConfig'} | ${'/'}          | ${`/${TEST_PROJECT_PATH}/ide_terminals/check_config`}
+    ${'checkConfig'} | ${'/gitlabbin'} | ${`/gitlabbin/${TEST_PROJECT_PATH}/ide_terminals/check_config`}
+    ${'create'}      | ${''}           | ${`/${TEST_PROJECT_PATH}/ide_terminals`}
+    ${'create'}      | ${'/'}          | ${`/${TEST_PROJECT_PATH}/ide_terminals`}
+    ${'create'}      | ${'/gitlabbin'} | ${`/gitlabbin/${TEST_PROJECT_PATH}/ide_terminals`}
+  `(
+    'when $method called, posts request to $url (relative_url_root=$relativeUrlRoot)',
+    async ({ method, url, relativeUrlRoot }) => {
+      gon.relative_url_root = relativeUrlRoot;
+
+      await terminalService[method](TEST_PROJECT_PATH, TEST_BRANCH);
+
+      expect(axiosSpy).toHaveBeenCalledWith(
+        expect.objectContaining({
+          data: JSON.stringify({
+            branch: TEST_BRANCH,
+            format: 'json',
+          }),
+          url,
+        }),
+      );
+    },
+  );
+});