Use new button styles

[antonok-edm]

Changes

Fixes #536.

Addresses concerns from #486 as well.

With this PR, buttons are all themed using a common Button component, and have a visual update as described in #536.

This will require changes in https://github.com/brave/brave-core wherever buttons are used as well.

Test plan

Storybook stories and snapshots have been updated to use the new buttons. I've done a scan through all of the usages of buttons to verify that everything looks acceptable, but another pair of eyes would be appreciated.

Link / storybook path to visual changes

• Primarily https://brave-ui.alazarev.now.sh/?path=/story/components--button, but most of the stories include some kind of button and have also been updated accordingly.

Integration

• [x] Does this contain changes to src/components or src/

  • [ ] Will you publish to npm immediately after this PR, or wait until sometime in the future?
  • [x] Incompatible API change to something existing (major version increase)
  • [ ] Adding new backwards-compatible functionality? (minor version increase)
  • [ ] Fixing a bug backwards-compatibly? (patch version increase)

• [x] Does this contain changes to src/features for brave-core?

  • [x] Are there non backwards-compatible changes required for brave-core? Do not merge until brave-core PR is approvable. Link to brave-core PR:
  • [ ] Will you create brave-core PR to update to this commit after it is merged?
  • [ ] Wants uplift to brave-core feature branch?
    • When uplift-approved, merge to brave-core-0.VV.x feature branch
    • Create additional brave-core PRs for each feature branch to update commit

[petemill]

Please add a storybook link via running now -p on your local branch.