Ui changes proposal

globalStyles:

• changed scrollbar height

Header:

• changes styles on Header and MainNav
• created two custom hooks on /_helpers and implemented them:
  • useKeyBoardOnList: to handle keydown events on userMenu and MainNav. reusable for any similar case
  • useWindowSize: to keep track of the window size
• refactored link rendering method
• renamed some of the StyledComponents
• added icons to userMenu

Headings:

• added a wraper for the title
• For the moment all the headings with level1 will have a linearGradient background

Table:

• changed styles

UserList:

• change styles to make on PageHeader and Table
• removed all related to selectAllCheckbox since it was added as a feature for List in previous MR

TeamManagerList:

• changed styles on PageHeader abd StyledSection