fix: button icon variant

[shibulijack-fd]

• Fixed buttons with icons.
• Fixed toast message button icon issue.

More info:
Button variant is not directly linked to icon variant. For instance, button of variant danger must display icon with secondary variant. Hence it requires a mapping.

Also, the toast-message component uses ember-cli-flash under the hood, which returns specific types such as "Info", "Warning" etc.. While we've followed similar naming conventions in our component, there exists a small problem of capitalisation. To fix this, there are 2 possible solutions:

Solution#1: Helper
Create a helper which capitalises the first letter.
Time complexity: O(n)
Space complexity: O(1)

Solution#2: JS Map
Create a simple mapper like:

const VARIANT_MAP = {
  'Info': 'info',
  'Success': 'success',
  'Warning': 'warning',
  'Danger': 'danger'
};

Time complexity: O(1)
Space complexity: O(n)

Although the helper seemed like a straight-forward solution at first, creating a modifier function is an overkill for a small component which does not even reuse this functionality. Hence, I'm going with the faster solution.