More flexible API for menu items

[nonprofittechy]

fix #723

Adds a new user-facing magic keyword: al_menu_items_custom_items which should be a list of menu items. Custom items are shown before default menu items.

Each menu items follows the Docassemble menu item format, which is a dictionary with the keys url and label. It can be convenient, and readable, to use a data from code block to define the menu items, but you can use any method to create the right Python datastructure.

Optionally, you can add a key hidden which contains a True/False value. If the value is True the menu item won't be shown. This can be used to dynamically show or hide the menu item on a given page. If the value is dynamic, you should add a reconsider modifier on the block that defines al_menu_items_custom_items.

E.g.,

reconsider: True
variable name: al_menu_items_custom_items
data from code:
  - url: |
      "http://example.com"
    label: |
       "Example link"
    hidden: user_turned_off_example_link

Note that if the visible state isn't dynamic, the author can use a data block instead of a data from code block. This will make the label translatable. Alternatively, the author can add a template for the label to make it available for translation.

[BryceStevenWilley]

LGTM. Getting this in the docs would be great too

[nonprofittechy]

LGTM. Getting this in the docs would be great too

On it!