Change Signup Login page behavior in non-custom domains

## Urgent

- [x] Update the domain name of free apps to `[project-id].authgear.cloud`
- [x] `authgear.cloud`, `authgear-apps.com` and `authgearapps.com` all redirect to `authgear.com`
- [x] The default free domain created on Authgear should not serve the signup login page on direct access.
- [x] If someone directly visit [project-id].[free domains] (e.g. `ovely-happy-123.authgear-apps.com`), show an error page with a branded error page, show the OAuth error.
- [x] At portal, remove the URL at top left; Remove the 1st block of Getting Started.
- [x] The default auth page should contain Authgear logo, but must also be different from the login page of Authgear Portal.

https://github.com/authgear/authgear-server/pull/3321

## Further Follow up
- [x] If someone visit an [project-id].[free domains].com which have a 404 [project-id] not found, show an Authgear branded error page say project-id is not a valid project, and try out authgear.com

https://github.com/authgear/authgear-server/pull/3323

## Future Cooldown / Pitch
- [x] Allow trigger "Try it out" from the portal (related to https://github.com/authgear/authgear-server/issues/3286)
- Serve a tester application at `[authgear-endpoint]/tester`
- The tester app cannot trigger authentication by itself
- The tester app offers a callback url to perform code exchange
- It will show:
- user info of the logged in enduser
- a button to open the AuthUI Settings page
- A button to go back to Authgear portal
- [ ] Add an option to enable custom domain + IdP initiated login
- Add an option to Custom Domain setup to enable directly showing signup / login page? (Call it IdP-initiated Login?)
- Add a How-To guides page at https://docs.authgear.com/ about `IdP-initiated login with Custom Domains`, and talk about the concept / how do they enable it.

## Re-Consider
- [ ] Do not allow changing the assigned subdomain: https://github.com/authgear/authgear-server/issues/3263