Add API key field to covidcast imports

[rzats]

Closes #47:

• Adds an API key field to the COVIDCast signal import dialog.
• This displays previously hidden private signals in the dropdown, making them available for visualization.

[rzats]

@melange396 also fixed!

[melange396]

This works, but it doesnt do its verification until the focus is moved off of the input field, and it doesnt repopulate the metadata until verified... The delay can be confusing and will be frustrating, unless perhaps we include a "working" indicator or similar. Did you explore triggering the key verification on typing (the on:input event instead of on:change)?

[rzats]

@melange396

This works, but it doesnt do its verification until the focus is moved off of the input field, and it doesnt repopulate the metadata until verified... The delay can be confusing and will be frustrating, unless perhaps we include a "working" indicator or similar. Did you explore triggering the key verification on typing (the on:input event instead of on:change)?

I moved the key verification to on:input with a debounce that makes sure it only activates once the user stopped typing for 500ms. The timer can be adjusted, try it out and see what you think!

[melange396]

One small comment. Otherwise, the oninput w/ delayed request looks great!

[melange396]

LGTM!