fix(GlLabel): Fix GlLabel close button

What does this MR do?

This MR fixes the GlLabel close icon. Since the close icon was wrapped in the label link, clicking on it redirects the page to the label link. This MR:

• Prevents clicking the close icon from redirecting to the label link
• Corrects the close button text and background colours including when hovering
• Refactors GlLabel to simplify the logic and to accommodate fixing the above two points

Video going through various states of the label:

Screen_Recording_2020-09-17_at_7.18.24_pm

Video showing how it looks in the issue sidebar:

Screen_Recording_2020-09-18_at_2.13.01_pm

Table of the different colour states:

bg color	scoped	span1 text	span1 bg	span2 text	span2 bg	button text	button bg	button text hover	button bg hover
light	no	black	bgcolor	black	bgcolor	black	bgcolor	bgcolor	black
dark	no	white	bgcolor	white	bgcolor	white	bgcolor	bgcolor	white
light	yes	black	bgcolor	black	white	black	white	white	bgcolor
dark	yes	white	bgcolor	bgcolor	white	bgcolor	white	white	bgcolor

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.