[Chore] Button Design Update

[grahamhency]

PR Checklist

• The commit message follows our guidelines: https://github.com/mobi/goponents/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

We have button variants for:

• positive
• negative
• neutral

Issue Number: Closes #170

What is the new behavior?

We have button variants for:

• primary (replacing positive)
• secondary
• tertiary
• neutral
• negative

Does this PR introduce a breaking change?

• Yes
• No

There are two changes that could be considered "breaking".

1. The alert variant is being removed. This was already deprecated in a previous version, but is officially being removed now.

• Any instances of alert that remain should be changed to negative

2. The positive variant is being removed, however, we are going to accept it as a variant and change it to primary for now. In this case, it's not functionality breaking, but possibly UX breaking.

• Not all positive buttons will need to be primary. In that case, the buttons labeled as positive should be evaluated on a per-component basis to determine if they need to be primary or instead should be secondary or tertiary.

Other information