Clarify what the invoiced_through field on a CustomerPlan indicates
after the changes to the billing state machine as ledger entries
are processed in invoice_plan.
Updates the plan activity page where this field is shown to support
admin users for these changes as well.
Follow-up to #34643.
Expands the header information on the plan activity view to include
the license ledger that was set as invoiced_through when the last
billing cycle generated an invoice.
Also, updates the name of the header information for the next
invoice date to match what's show in the support view for the plan.
Adds a link to the plan ledger view in the current plan information
shown in the support views. Link is not shown if the plan is 100%
sponsored, e.g., the Community plan.
Adds a formatted header area to the activity table template so
that it's easy to add useful information to these activity views.
