Add UI for configuring SAML via Flagsmith dashboard

[matthewelwell]

Currently, configuration of SAML is restricted to the django admin. We should add the option to configure it in the organisation settings in the Flagsmith dashboard.

This should only be possible for organisation admins, and is only available to enterprise customers.

The flow should be:

1. FE sends a request to /api/v1/auth/saml/configuration/ with the following information:

{
  "organisation": 1,  // id of the organisation to add SAML to
  "name": "org-short-name",  // a short name for the organisation, used as the input when clicking 'Single Sign-on' at login (note this is unique across all tenants and will form part of the URL so should only be alphanumeric + '-,_')
  "frontend_url": "https://app.flagsmith.com",  // this should be the base URL of the Flagsmith dashboard
  "allow_idp_initiated": true,  // this field determines whether logins can be initiated from the IdP
  "idp_metadata_xml": "...",  // the metadata from the IdP (this should not be required on creation as it might depend on our service provider metadata)
}

Once this is created, it can be updated / deleted using the relevant HTTP verbs and the URL /api/v1/auth/saml/configuration/:name/.

2. The FE offers the option to download our service provider metadata XML which can be obtained using the endpoint at /api/v1/auth/saml/{name}/metadata/.

3. Finally the FE should offer the option to upload the IdP metadata in response, as per the description above.

There are additional entities that can be used for mapping fields between the IdP and Flagsmith, but these do not yet have endpoints. Once these are ready, we will update this issue or create a new one.