fix 2 ARIA problems with speaker pages

[phette23]

This addresses two accessibility problems on the speakers and past keynotes pages.

1. the aria-expanded attributes of the speaker name links does not toggle, it simply stays "false" all the time even when a speaker's info box is displayed
2. aria-controls attributes target non-existent elements

See MDN for aria-expanded reference. It's my understanding that the control must reflect the state of the controlled element, but the controlled element itself does not need any particular ARIA properties.

To test:

• apply the PR
• visit the past keynotes page http://127.0.0.1:4000/speakers/past-keynotes
• inspect the HTML of the name of a particular keynoter
  • their aria-expanded attribute should be "false"
  • their aria-controls value should match the id of the corresponding .speaker-info box beneath
• click the keynoter's name link
• their aria-expanded attribute should now be "true"
• click another keynoter's name link
• the original speaker's aria-expanded attribute should be "false"
• the new speaker's aria-expanded attribute should be "true"

Since these changes are to a speaker_box.html include they should fix both the past keynotes page and the regular speakers page http://127.0.0.1:4000/speakers/ but if you want to test that page, you first need to copy our example data (_data/examples/speakers.yml) into the _data directory.