Update arrows to nicer looking ones

[Former user]

The current arrows on the data visualisation are not very nice to look at - we should use more modern, smaller ones. Maybe we can get inspiration from other websites that use these kind of navigation bars.

Maybe a simple triangle will already improve the looks

[Enrico Piccini]

I think for now it's ok. This is a bootstrap icon. We will improve the design in the final version of Udemi