fix(batches): the baseURL for github instance is now updated when creating a GitHub app (#63803)

Closes SRCH-723

The baseURL for GitHub apps defaults to `https://github.com` when no
`externalServiceURL`, we somehow missed this during our testing.

![CleanShot 2024-07-12 at 11 57
00@2x](https://github.com/user-attachments/assets/99b68a11-de38-4a2d-8c4c-3219f0c9abf7)


## Test plan

<!-- REQUIRED; info at
https://docs-legacy.sourcegraph.com/dev/background-information/testing_principles
-->

Manual testing with the GHE instance.

## Changelog

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client/web/src/enterprise/batches/settings/AddCredentialModal.tsx

@@ -383,6 +383,7 @@ const AddToken: FC<AddTokenProps> = ({
                         authenticatedUser={user as unknown as AuthenticatedUser}
                         minimizedMode={true}
                         kind={kind}
+                        externalServiceURL={externalServiceURL}
                     />
                 )}
             </>

client/web/src/enterprise/batches/settings/BatchChangesCreateGitHubAppPage.tsx

@@ -23,16 +23,18 @@ interface BatchChangesCreateGitHubAppPageProps {
     authenticatedUser: AuthenticatedUser
     minimizedMode?: boolean
     kind: GitHubAppKind
+    externalServiceURL?: string
 }
 
 export const BatchChangesCreateGitHubAppPage: FC<BatchChangesCreateGitHubAppPageProps> = ({
     minimizedMode,
     kind,
     authenticatedUser,
+    externalServiceURL,
 }) => {
     const location = useLocation()
     const searchParams = new URLSearchParams(location.search)
-    const baseURL = searchParams.get('baseURL')
+    const baseURL = externalServiceURL || searchParams.get('baseURL')
 
     const isGitHubAppKindCredential = kind === GitHubAppKind.USER_CREDENTIAL || kind === GitHubAppKind.SITE_CREDENTIAL