Add option to control position of links and dropdowns in navigation.

[linuspahl]

Description

Currently the order of navigation items is based on the following logic:

All navigation items are part of the plugin store. The items from core get registered first and the items from plugins afterwards. Simple links are rendered first and dropdowns second. The rendered order of e.g. pages is based on the order in the plugin store.

The only way to control the position of a dropdown is via the attribute position: 'last'. This is currently being used for the system dropdown.

It would be helpful to have more control over the order of navigation items. For example to place navigation items from a plugin in between navigation items from core.

Graylog Version: 6.0