feat: implement SHOW_REGISTRATION_LINKS setting

[CefBoud]

Description

This PR implements the newly added SHOW_REGISTRATION_LINKS setting whose purpose is to hide the public registration functionality without disabling the register API.

ALLOW_PUBLIC_ACCOUNT_CREATION is already taken into account in the frontend-app-authn and is expected to be set through the runtime configuration.

This PR introduces the possibility of adding the SHOW_REGISTRATION_LINKS setting to the MFE runtime config so it can be taken into account for hiding the register functionality without disabling the API, thus replicating the legacy LMS flow.

How Has This Been Tested?

The steps to test are as follows:

1. LMS configuration:
   • Set ENABLE_MFE_CONFIG_API to True in the LMS settings
   • Set SHOW_REGISTRATION_LINKS to False in the MFE_CONFIG dict in the LMS settings
2. frontend-app-authn configuration:
   • set MFE_CONFIG_API_URL to 'http://localhost:18000/api/mfe_config/v1' in .env.development to enable runtime conf
3. Launch LMS + frontend-app-authn in Devstack
4. Head to http://localhost:1999/login. There should be no register tab.

NB: mind the MFE_CONFIG_API_CACHE_TIMEOUT setting if you want to play around with MFE_CONFIG. The result is cached for 5min by default. The runtime config also relies on client-side caching, so better to rely on private browsing if there is a config change.

Screenshots/sandbox (optional):

Include a link to the sandbox for design changes or screenshot for before and after. Remove this section if its not applicable.

Before	After

Merge Checklist

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Post-merge Checklist

• Deploy the changes to prod after verifying on stage or ask @openedx/vanguards to do it.
• 🎉 🙌 Celebrate! Thanks for your contribution.

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