More declarative method for defining navigation sections

[nonprofittechy]

Fix #262

like #726 does for the drop-down menu, this creates a method to control the navigation items on the left in a more declarative way. The author can provide datastructure that is recalculated fresh on whatever basis makes sense for the interview (reconsider modifier, inline call to reconsider, or depends on modifier as appropriate). The datastructure will list every potential menu item and then put the rule for turning the item on or off in the optional hidden key.

Because we have a lot of existing interviews that use the simpler sections syntax to define static sections, this behavior is turned off by default.

Example:

---
code: |
  enable_al_nav_sections = True
---
reconsider: True
variable name: al_nav_sections
data from code:
  - item1: |
      "Label1"
  - item2: |
      "Label2"
  - item3: |
      "Label3"
    hidden: _internal.get('steps') < 2