default tiles set to map instead of sat
core.cms: permit 1 level nesting #172

Improvements to nodeshot.core.cms in preparation for #172 (UI: implement
dynamic menu).
* improved admin
* improved model by permitting 1 level nesting
* added possibility to specify css class for menu item
* db migrations
core.cms: reflect nested structure in API

preparation for #172
UI: menu items taken from API, closes #172

Menu items are taken from the API, so the main menu can be built by
admins from the admin web interface.