new-calendar-page: Change timing of URL validation feedback, allow cancelling while typing
Note from Jeff: should be merged after !423 (merged), preferrably; then this MR here will need to be rebased & resolved. It is also easier/cleaner to test on top of !487 (closed).
Currently, the progress bar is pulsed as soon as the user starts typing, giving the impression that a process is on going. But, in reality, validation is performed 500 ms.
After this changes the progress bar is pulsed only when validation is on-going. The validation timeout was increased to 750ms too, so the users have more time to type without prompting validation.
Also, the user can now cancel the validation/discover by typing into the entry, in case they made a typo, took a pause, or want to delete it, expecting a cancelled operation.
Additionally, the entry progress bar is reset after the operation is completed, cancelled or it fails, so the progress bar stays hidden when it is not necessary.
Fixes #1272 (closed) and #1273 (closed)