[payamnj]

Summary

Adds ORGANIZATION.CUSTOM_TABS so a deployment can show its own tabs on an organization's page. These are for settings that belong to the organization but not to the library, such as a payment provider connection. Inbox Academy will use it for a Stripe Integrations tab, which Stripe's app review requires.

• Each tab is a custom element given by NAME, LABEL, SCRIPT_URL, STYLE_URL and COMPONENT_TAG. This is the same shape as the sidebar and dashboard custom components.
• Custom tabs come after the built-in ones. ?tab=<NAME> opens one, so it can be linked to. A name that clashes with a built-in tab (general_info, members, newsletters, api_keys) is ignored.
• The tag goes through the same sanitizeComponentHtml as the dashboard components, which strips event handlers and scripts. The tab's script and style URLs go into the same deduped list in the page head as the navbar and dashboard components.
• To build the tabs per organization, override SingleOrganization.get_organization_custom_tabs(organization), for example to put the organization's id on the tag.
• Documented in docs/source/installation.rst and the CHANGELOG. Bumps the version to 7.21.0.

Testing

• pytest tests/platform: 628 passed. The coverage gate fails on this subset run only.
• New view tests: no tabs by default; tabs are read from settings, with their assets added to the page head.
• vitest Organization.test.jsx: 16 passed, including 4 new tests. They cover rendering a tab and its component, opening one from ?tab=, ignoring a name that clashes with a built-in tab, and stripping event handlers.
• ruff, ruff format, mypy and eslint are clean.

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