fix(button): refactor button component

[PeterShershov]

Refactored button component and updated tests.
div to button, em to rem.

@johanzilber @liorheber

[AmirAsaraf]

@PeterShershov why do you need the button implementation over a simple div? It causes misalignment with the dropbox (fonts and default button stuff)

[PeterShershov]

I fixed the alignment by setting the font-size to be similar to the dropdown.
Changing the div to a button will help with semantics, accessibility, html attributes such as disabled will prevent onClick to trigger automatically... @AmirAsaraf

[liorheber]

Well, button in Firefox has an added animation on click which will make it jitter.
You will need to align the styles between the browsers