Improve keyboard navigation around exercise component

[gadenbuie]

Currently, the exercise component traps keyboard focus when a user enters the code editor. The goal of this PR is to alleviate this problem by allowing users to press Esc to turn off the use of the Tab key for indent and outdent behavior. We also add an outline to the editor element when in this mode to visually indicate that focus has been "given back" to the container. Note that indent/outdent via Ctrl + ]/[ will still work in this mode.

Additionally, I replaced the use of anchor elements with class btn in the exercise toolbar with <button> elements. This automatically allows those buttons to receive keyboard focus as well.

Finally, a small change was needed in the autocomplete logic to ensure that the Tab events aren't trapped when they're intended to move focus out of the exercise element. In this process, a small bug with the autocompletion on empty code was revealed: we no longer will try to return autocomplete suggestions on an empty line.

For #636

Here's a screen recording showing the new behavior. I added a red focus outline to make it more obvious which object has focus.

1. Tabbing in the tutorial moves through the exercise buttons — Start Over, Run Code — and then into the exercise editor.
2. Here, Tab inserts an indent when pressed at the start of the line and Shift + Tab inserts an outdent.
3. When code has been written on the line, Tab opens autocomplete.
4. Pressing Esc disables indent/outdent via Tab, allowing the user to move focus to the next or previous element.
5. Users can also trigger the button behavior using the keyboard for both Start Over and Run Code.

learnr-exercise-tab-behavior.mp4