Overhaul the ACP form to import languages

[dtdesign]

• [ ] Disallow the creation of unknown phrases, these must be silently ignored.
• [ ] Remove the selection of the package the uploaded file belongs to.

Motivation for these changes:

This specific feature has existed for literally decades and used to be a simpler way to add or update phrases during development. This has long been replaced with the sync feature of the built-in developers tools that allow one to import phrases without having to build an update package.

The problem is that this uses the same internal API used for importing phrases from packages which means that it is allowed to add new phrases. This is why we added the package selection at some point to properly assign new phrases to that package instead of the core, which breaks the ability to update them.

And this is exactly where the problem lies: This feature is almost exclusively used to import localizations which by definition should never add new phrases and instead only provide values for existing phrases. Changing the behavior would allow us to drop the package selection because the correct package id would be implicitly resolved with every unknown phrases silently being ignored.

See https://www.woltlab.com/community/thread/293093/