build: bump react-popper to version 2

[xballoy]

Bump react-potter to latest version. I had to add @popperjs/core as dependency. See react-popper migration guide.

I refactored the unit tests of Popover to have functional tests so they do not depend on the implementation.

Changes :

• data-placement becomes data-popper-placement so it might break CSS if people are overriding the default behavior
• the arrow design in now in a ::before pseudo-element because Popper uses transform to position the arrow inside the popper

To override the design of the arrow you need to write the following CSS:

  .af-popover__container-pop {
    &[data-popper-placement^='top'] > .af-popover__arrow::before,
    &[data-popper-placement^='right'] > .af-popover__arrow::before,
    &[data-popper-placement^='bottom'] > .af-popover__arrow::before,
    &[data-popper-placement^='left'] > .af-popover__arrow::before {
      // Override background or write your custom CSS
      background: white;
      box-shadow: -2px 0 5px 0px rgba(0, 0, 0, 0.2);
    }
  }

@guillaume-chervet @samuel-gomez @youf-olivier

[xballoy]

I had to force the version because I don't know why but it always downloaded a newer version which breaks the modal package.

[xballoy]

The react-test-renderer version wasn't aligned with react and react-dom

[xballoy]

The arrow is now in a ::before pseudo-element so I rewrote it so it's now easier to override

[youf-olivier]

I Merged an older PR before this one, and so you have some conflicts

[xballoy]

I updated the PR to solve the conflicts: rebase on master and remove the reference to AXA Nexus

[youf-olivier]

Holly s... seriously ?