EZP-20424 - As a developer, I want eZPage field type working in read only mode

See [corresponding Jira issue](https://jira.ez.no/browse/EZP-20424).
To test with DemoBundle, check https://github.com/ezsystems/DemoBundle/pull/2

## Description
Goal is to have **Page** fieldtype working in read only mode, with an sub-controller as a replacement of the legacy `block_view_gui` *eZTemplate* function.

Note that the cronjob is still handled in the legacy stack :
```bash
php ezpublish/console ezpublish:legacy:script runcronjobs.php ezflow
php ezpublish/console ezpublish:legacy:script runcronjobs.php ezflow-cleanup
```

## TODO
- [x] Refactor current field type implementation to use `ezm_pool` table for pending/valid/archived block items.
- [x] Add tests for the newly refactored classes.
- [x] Add semantic configuration available by siteaccess when needed (TBD by @dpobel).
- [x] Implement the default view template block ( *Twig* ).
- [x] Maintain BC with legacy configuration (zone templates configured as `*.tpl`
- [x] Add an internal controller for rendering blocks to replace the legacy `block_view_gui`.
- [x] Implement a new `BlockViewProvider`, responsible for selecting the appropriate template for a given block.
- [x] Implement a `BlockViewProvider` for *legacy* (using `block_view_gui`)