Don't ask for code review after task #1

(Task #2 is very simple; merge code reviews after #2)
Add a default message that is persistent.
Remove space in textarea.
Switch task #3 from filter -> clear.

Filtering was distracting from the core focus on Flask/JS/sessions,
so swapped for a new task that will let them practice HTML/Flask.
Minor style cleanup.
Add ID to submit button.
Add hints about throttling..