reCAPTCHA custom template

This issue has been migrated from [#15501](https://github.com/matrix-org/synapse/issues/15501).

---

### Description

Testing templates i noticed that recaptcha template does not honour the custom templates directory. The logo was different.

### Steps to reproduce

- Create a custom templates directory
- Include _base.html template
- Make some changes on _base.html template, like logo.
- define the custom template directory on homeserver.yaml
- Test the recaptcha template

### Homeserver

Private

### Synapse Version

{"server_version":"1.82.0","python_version":"3.9.2"}

### Installation Method

Debian packages from packages.matrix.org

### Database

Single PostgreSQL, no, no

### Workers

Single process

### Platform

Azure VM,
Standard_D2s_v3 Size,
vCPUs: 2
RAM: 8 GiB
Operating system: Linux (debian 11) x64

### Configuration

_No response_

### Relevant log output

```shell
No relevant error messages.
```

### Anything else that would be useful to know?

SSO OIDC Template

RECAPTCHA Template