[Subsidies] Allow for internal notes on subsidy objects

Feature Request

Description

In the subsidy creation form (https://scipost.org/finances/subsidies/add/), include an optional field for internal remarks visible only to Sponsors Administrators (might be called Financial Administrators?).

• The remark should be visible on the subsidy details page https://scipost.org/finances/subsidies/[XXX]/ and have the option to edit it.
• When viewed on https://scipost.org/finances/subsidies/, subsidies for which a remark exist should be marked with some icon (only for SponsAdmin/FinAdmin).
• A similar icon should appear for SponsAdmin/FinAdmin users when viewing the subsidy on the list on an organisation's page https://scipost.org/organizations/[XXX]/.