Fix: forget a closed action's cached schema when it is unmounted

[webard]

Description

unmountAction() pops the action off mountedActions, but leaves its schema in cachedSchemas
under mountedActionSchema{nestingIndex}. getMountedActionSchema() reads that cache before it
builds anything, so an action mounted at the same nesting index later in the same request is handed the schema of
the action that has just closed.

This causes actions to have their own header and footer, but someone else’s form. The bug appeared while testing the modalClickThrough() method. It occurred when I opened one action and then, without closing the current one, opened another.

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.

[webard]

I realize that the same fix can be achieved in userland with the replaceMountedAction() method, but that is not entirely intuitive, and not handling this automatically introduces a silent bug.

However, if replaceMountedAction() is intended to be the only way to fix this problem, I fully agree with that and will close the PR.