Replace sort caret with default SS styling

[brynwhyman]

The frontend table is using a sorting icon provided by Griddle.

This should be:

• Replaced with the caret with provided with the Wātea theme something more in line with SS styling
• Ensure the icon is always present on the column heading to highlight that the column is sort-able. (Griddle won't show the icon until the column heading is clicked)

[robbieaverill]

Replaced with the caret provided with the Wātea theme

We should be careful not to couple it though. The theme isn't a dependency of this module.

[ScopeyNZ]

Agreed. This issue should probably read "use default SS styling" or something. Preferably Wātea should not require any updates for this.