[Mailing List] UX Improvements & Consistency/Security Guards

Related Tickets:
#2282
#2283
#2436

This ticket covers improvements to the Mailing List subscription flow.
Ref Slack [thread](https://metalab.slack.com/archives/C09K0KC9GTB/p1780000624141299).

1) If a user attempts to subscribe an email that's already subscribed by a different account, the current message says " is already subscribed to this list". This doesn't follow data compliance best practices, as this could be used to map which emails are subscribed to these lists. Not priority, but should be looked into.

2) In the subscription process, the FE business rules won't allow a user to subscribe to different mailing lists using different emails. We should add a guard for this to the BE as well. This will happen at a later point when we add the management component in the user profile edit section, so this is just a reminder actually.

3)
> When user already has at least 1 confirmed subscription and only 1 pending – I think the phrase verify ownership of this address  no longer make sense (it only does when user has 0 active subscription). Perhaps we should distinguish this case?

We should adapt the phrase in this case to be more explicit about the current state of the users subscriptions.

4) Automatically populate the subscription modal if the user is already subscribed to mailing lists with that email (subscription made through Postorious).

5) Skip mailing list email validation if the user is using any already validated email (primary email, commit emails etc.).

6) Add proper error page, instead of leveraging the success one with errors attached;

7) Add a diagnostics function check that fetches the API with a read-only query like "/api-proxy/3.1/lists";