[proposal] unified export format

# problem
- Currently have to use several different forms to do both export and then import
- You also have to know the right order to do the import when you have multiple export json

# proposed solution

User will go to the site setup and click on export and get a UI something like

```
-------------------------------------------------------------
| Warning: multiple exports selected. Download will be tar.gz |
-------------------------------------------------------------

# Exports
[x] Content
[x] File/Images
[ ] Users
[ ] Content Tree
[ ] Relations
[ ] Translations
[ ] Local Roles
[ ] Default Page Mapping
[ ] Object Positions in Parent
[ ] Comments

# Content Export

{query widget}
Type: Page
Path: /news depth:1
Path: /other-news depth:1
Creation date: > 1/1/20018

Selected content (21 items)
--------------------------------
| /news/item1
| /news/item2
| /other-news/big-news
--------------------------------

# File/Images Export

(o) url/path
( ) binary in tar.gz
( ) base64 encoded in json

[Download] [Save to Server] [Dry Run] [Cancel]
```

Features this adds
- tar.gz if multiple exports selected
- still keeps json as multiple files
- Would enable things to be put in the right orde
- still memory efficient as tar can be streamed.
- somehow take every exporter and merging into one UI.
- Adapter to get schema?

# Alternatives considered
- still have seperate views?
- use a single json format that contains the others
- can still stream it and webserver might do gzip compression automatically anyway
- can still choose to include blobs for small sites using base64 encoding.
- con: might be harder for the user to inject or change blobs without writing code?

Start by reviewing the existing export and import views and exporters behind the site setup flow; the issue does not name specific files or tests. Compare how content, files/images, users, and other listed data types are currently handled. Done would require a unified selection UI, ordered multi-export output, and tar.gz or JSON handling as described.

Written by the indexing model from the issue text.