Menu manager

[mutantsan]

I've started to work on this feature already. The idea is simple:

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Pitch with dev team required

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Meta issue that should be planned and split

1. Sysadmin can create multiple menu entities.
2. The menu entity can contain multiple menu items with different attributes (such as title, url, class etc)
3. The order and hierarcy of menu items can be changed via drag'n'drop mechanism.
4. There is a page to list all existing menus
5. There is a page to create a menu
6. There is a page to manage menu items for specific menu
7. There is a helper and/or snippet that helps you to render a menu by its unique name
8. Menu is styled responsively
9. Idea: Can we fetch the menus from a remote source, like Drupal API or maybe even scrap it from somewhere and cache? Do we need some kind of interface to transform the data so it fits our format?
10. Question: should it be a part of the blocksmith extension or a separate one?
11. Question: should we merge the blocksmith extension into the admin panel?