css: Fix width and alignment

On 3.30 cycle we've changed a few styles based on old mockups. Once that was deployed and users started to see it, users started to complaint about notes width being too small (#104 (closed)) and notes justify alignment making reading and writing notes harder (#102 (closed)).

After discussing those issues with Design team, @aday and @bertob, we decided to revert alignment to left and to increase width to around 700 pixels using default font and theme. @aday also asked to use ems instead of pixels, so notes width will change according to font size.