User Subscriptions feature starting point

[danguita]

Connects to #166.

What does this PR do?

This PR introduces a new feature which allows any signed in User to subscribe to determined system resources in both generic and specific contexts. To do that, we're adding a new model called User::Subscription to the system and a simple interface to access it.

It also allows any not signed in User to subscribe to those exposed resources by starting a new User registration process with the pre-created User::Subscription item.

Extra 🎱

• We are preparing the User namespace here by trying to keep the default layout generic and extracting a new one for User namespace only.
• There has been some progress in the User namespace localisation process, and any new implementations are being localised as well.
• We are taking care of the "every-interaction-has-a-form-object" approach by covering any missing scenarios around there, and making those form objects contain most of the model logic instead of the ActiveRecord models itself.

How should this be manually tested?

As an integration sample, the GobiertoBudgetConsultations::Consultation model has been set up as a the only "subscribable" resource for now, so these are the available user interactions:

A. When signed in as a User

The subscription box should be shown as follows:

Generic context URL: http:https://madrid.gobierto.dev/consultas_presupuestos/

Specific context URL: http:https://madrid.gobierto.dev/consultas_presupuestos/<consultation_id>

B. When not signed in

The subscription box should be shown as follows:

Generic context URL: http:https://madrid.gobierto.dev/consultas_presupuestos/

Specific context URL: http:https://madrid.gobierto.dev/consultas_presupuestos/<consultation_id>

✏️ Known improvements

Finally, I'm keeping some known improvements here for future iterations:

• Detect when a signed in user has already subscribed to the current resource, and show an "Unsubscribe from this" button instead.
• Perform any subscribe/unsubscribe actions asynchronously to avoid the annoying redirections.

[ferblape]

Looks very promising! Merging after the catalan magic.

[danguita]

Yo, thanks for that! 👏

[danguita]

The build has just passed in master branch. I'm rebuilding this one's for mental sanity.