Replace the progress bar with a spinner

[mirkoperillo]

Currently Resting provides a progress bar with the goal of visually communicate that a request is in progress.

The progress bar is suitable when there is a clear way to calculate the progress of a operation.

In the case of a HTTP request this could be useful for a download request or something like this. This type of requests are not the common case of usage for Resting.

I think it could be better to substitute the progress bar with a spinner visualized somewhere in the buttons area.

The motivations are:

• Progress bar consumes space that can be better used by the request and response panel
• Progress bar needs logic in multiple parts of code-base: https://github.com/mirkoperillo/resting/blob/52d009e0950084b220a0dad3179f6e91aed7d879/src/js/app.js#L83 https://github.com/mirkoperillo/resting/blob/52d009e0950084b220a0dad3179f6e91aed7d879/src/js/app.js#L142 https://github.com/mirkoperillo/resting/blob/52d009e0950084b220a0dad3179f6e91aed7d879/src/css/resting.css#L228 https://github.com/mirkoperillo/resting/blob/52d009e0950084b220a0dad3179f6e91aed7d879/src/index.html#L160