[FIX] Dropdown elements were using old styles

[kaiiiiiiiii]

Closes my promise to @karlprieb.

This PR changes all dropdown element types from rc-input__element to rc-select__element.

Two more things:

1. The dropdown has still a cut off at the bottom (like any other dropdown in RC). May someone can give me a hand with that? :)
2. Is it possible to remove the margin ("0.5rem 0") from the rc-select element? Looks a lot better without it.

Before (User UI):

After (User UI):

Before (Admin UI):

After (Admin UI):

[karlprieb]

@kaiiiiiiiii can you add this arrow icon like we did on the RocketChat Apps admin panel?

Thank you! 👍

[kaiiiiiiiii]

@karlprieb Done :)

[karlprieb]

Thank you @kaiiiiiiiii :)

Can you add a screenshot about the cut off at the bottom that all select elements have?

[kaiiiiiiiii]

100% zoom

400% zoom

1600% zoom

The UI gets a bit messy with zoom, but it's really hard to see the cut off without it.

[karlprieb]

@kaiiiiiiiii you can put line-height: 1rem !important; on .rc-select__element class for now and we will find a better way to fix that in the future.

[kaiiiiiiiii]

Done that as well, mate :)