Some accessibility items

Some accessibility items that need addressing. For standard keyboard interaction, see https://webaim.org/techniques/keyboard/.
- [ ] More contrast needed for hover and other interactions, including (now issue #852):
- [ ] General nav bar
- [ ] Form section nav bar
- [ ] Spouse vs. other member drop down in 'Household' form section
- [x] More visible `:focus` styling. [Being stewarded by knod right now]
- [ ] Income inputs are waaaaaay too much to tab through. We need to figure out something with arrow buttons... somehow... Really not sure how. Maybe we need to break them up into different sections and be able to skip to the next section somehow? :/
- [ ] Spouse/member drop down sometimes doesn't drop when user navigates to it.
- [ ] Spouse/member drop down chosen item sometimes doesn't switch when it should.
- [x] ~The buttons on the home page seem to get tabbed over twice.~ [Now #841]
- [ ] When a modal is open, things underneath still get tabbed over (not sure if that's fixable).
- [x] The new focus style from #811 isn't working within modals. [knod]
- [x] Do labels need to have a `for` attribute that'll connect them to their ui element in order to be read properly by a screen reader? [yes]
- [ ] Add a `for` attribute to labels to match their form input names. I think for `ManagedNumberField` we can give the same name to all the inputs. Worth a try.
- [ ] Add aria labels and descriptions (https://www.w3.org/WAI/tutorials/forms/instructions/)