Addons: allow users to opt-in into the beta addons (#10733)
* Addons: allow users to opt-in into the beta addons
I created the URL, models, view and form to prepare ourselves for the beta
addons. Currently, if the `Project.addons` fields is not null we add a HTTP
header to tell Cloudflare to enforce the new beta addons on this project.
In the future, we can use each of the `ProjectAddonsConfig` field to let the
user decide which of the addons they want to enable/disable and change their
configurations.
* Related: https://github.com/readthedocs/ext-theme/issues/212
* Changes from feedback
Use a `AddonsConfig.enabled` field to decide whether or not all the addons are
enabled.
* Docstring to document the HTTP headers to communicate with CF
* Addons: update modelling and queryset
* NGINX: pass the `X-RTD-Force-Addons` HTTP header
* Declare the URL only when ext-theme is enabled