Provide better download button style

I see a few different button styling issues floating around the issue queue, so not sure if the maintainers are doing a round up at some point? I did make this little patch that:

1. Converts the anchor to a button
2. Adds Drupal styling to the button
3. Removes the icons (I don't think they particularly lend a lot and just customizing the button attributes works just as well

I make an issue fork and am working on trying to add it there. But, in the meantime, here's a patch file.

Just made a merge request!

Also, I realized there was some weirdness happening with the first patch when it tried to delete the png files. I'd get a note in the `.patch` file with something like `bin differs from dev/null`. This was while generating a patch file from a cloned version on my local, so it might have been a weird permissions error since I was deleting the entire icons directory?

I pushed up my commit with the changes listed above, made a merge request, and re-downloaded the patch from the merge request page. Unfortunately, it still seems to be struggling to figure out deleting the png files, so might be better to omit the lines related to deleting the png files for folk applying the patch :/

Fine.
Now button is better and more visible.
Thank you

Update: We're testing this on our website and realizing there is some weirdness happening when the view display the button is attached to ALSO has filter views. We're testing on Chrome and, when the Apply button for the filters is pressed, the logic on the Download CSV button is also executing. We're troubleshooting now, but will push up a fix once we find a solution.

Ope. I see this is actually an existing issue. https://www.drupal.org/project/views_data_export/issues/3341736 🐛 "Apply" exposed filters button starts export instead of filtering Active