remove the tabindex attr from non-interactive DOM elements

These changes follow guidance from Sandi and from the general DOM specification that the `tabindex` attribute (along with Angular's `isFocusable` and `isTabbable` attributes) should only be included on elements that can be interacted with via mouse clicks or keyboard input.

A number of elements in the degree planner view were non-interactive by design but still were in tabbable. This commit removes tabbing from those non-interactive elements.