Disable add/remove mailbox buttons based on user permissions

[fredrikekelund]

Proposed Changes

For sites with multiple users, only the owner of the Titan/Google Workspace subscription is allowed to add/remove mailboxes. Currently, there is no preemptive warning to users about this. This means that they can click the button to add/remove mailboxes, but the action will fail.

With this change, the "Remove mailbox" button is hidden entirely and the "Add new mailboxes" button is disabled if the current user isn't allowed to perform those actions. We also display an explanation in EmailPlanHeader for why the buttons are disabled.

Testing Instructions

1. Have a site with at least two users and a Titan/Google Workspace subscription. The secondary user should be an administrator.
2. Log in as the secondary user
3. Navigate to /email/:domain/manage/:site_slug
4. Verify that the action menu for each mailbox does not contain a "Remove mailbox" button
5. Verify that the "Add new mailboxes" button is disabled
6. Verify that there is a warning text in the header that says Only the email subscription owner can add or remove mailboxes.

Related to #

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-35175

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-35175&env=jetpack

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~102 bytes added 📈 [gzipped])

name   parsed_size           gzip_size
email       +431 B  (+0.1%)     +102 B  (+0.1%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[fredrikekelund]

This is a minor additional change that I snuck in here. Here are two screenshots that illustrate the difference it makes when the action menu is initially opened.

[fredrikekelund]

Given that it's now possible that two .email-plan-warnings__warnings are displayed simultaneously, I found it appropriate to reduce the spacing between them slightly. Here's a screenshot of the result:

[fredrikekelund]

This is a styling fix that prevents overflow trouble with the action menu on smaller screens (e.g. laptops). Using bottom left matches the behavior in the DNS editor. Here are two screenshots that illustrate the difference this makes (notice the vertical viewport scrollbar in the first screenshot):

[stephanethomas]

I had minor suggestions but otherwise this looks good, and works great.

[a8ci18n]

This Pull Request is now available for translation here: https://translate.wordpress.com/deliverables/7366362

Thank you @fredrikekelund for including a screenshot in the description! This is really helpful for our translators.

[a8ci18n]

Translation for this Pull Request has now been finished.