Count pending ownership sites towards account usage on the upgrade page

[RobertJoonas]

Changes

When the user goes to upgrade to a plan, currently we let them upgrade to a suitable plan based on the usage across their owned sites. When the user has pending ownerships however, they might not be able to accept them, even after upgrading to a seemingly suitable plan, which can be annoying/confusing.

Whenever there are pending site ownerships, a message will be shown to the user:

Counting the usage from pending ownerships means that we are not letting them upgrade to a plan that would not accommodate all the pending sites. Same goes for warning about losing access to a feature used be a pending ownership site.

Tests

• Automated tests have been added

Changelog

• This PR does not make a user-facing change

Documentation

• This change does not need a documentation update

Dark mode

• This PR does not change the UI

[RobertJoonas]

This went through a bit more extensive refactor. I imagine it's pretty difficult to grasp at first sight so let me try to break it down a bit:

• We don't actually need the site option - this was only used to calculate the usage of the "to be transferred" site, which was added to the user's current team member usage to get the "usage after transfer". The same can be achieved through the pending_ownership_site_ids option.
• When querying memberships, we will first count the owner too, and later filter that out by exclude_emails which always contains the given user. That's because for pending ownerships we do want to count owners (after ownership is accepted by the user, the previous owner will remain an admin of the site)