Styles in progress_bar.less do not match progress bar styles elsewhere

[NikitOS]

Affected version

2.2.15, 2.3.0 Beta 2

Progress bars in polls and attachments do not have a border. I don't think it should be here either. Also, the border-radius value should be replaced with @xf-borderRadiusMedium, as done in the polls: it looks much nicer. And in polls and attachments the progress_bar macro should be used.