Use SingleSelect

[karlijnbok]

Context

After some issues arose due to refactoring of the css, it was decided that the select2 implementation in the SEMrushCountrySelector component should be replaced with a react-select component from the monorepo (see javascript#851 for the adjustments made in the monorepo).

IMPORTANT: the styling of the SEMrushCountrySelector component is not completely correct yet (i.e., the placement of the button next to the Select, it is currently still below the Select). An issue has already been added to the board of Team Frontend.

Summary

This PR can be summarized in the following changelog entry:

• Uses react-select instead of select2 for the SEMrushCountrySelector component.

Relevant technical choices:

Test instructions

This PR can be tested by following these steps:

• Checkout P3-144-adjust-multiselect on wordpress-seo and the monorepo
• Run yarn link-monorepo and build free
• Edit a post and delete the errors in the console as they are not related
• inster a ckeyphrase and click the "Get related keyphrases button"
• Make sure the styling of the select is correct
• Make sure the search function works
• Make sure the countryCode value in the store is updated when you change the country
• check the console, see that a warning is reaised but it depends on the internal implementation of the SingleSelect component so it's not due to our PR (if you open the Advanced accordion item below, which uses the ReactSelect as well, you'll see the same warning):

Warning: Using UNSAFE_componentWillReceiveProps in strict mode is not recommended and may indicate bugs in your code. See https://fb.me/react-async-component-lifecycle-hooks for details.
* Move data fetching code or side effects to componentDidUpdate.
* If you're updating state whenever props change, refactor your code to use memoization techniques or move it to static getDerivedStateFromProps. Learn more at: https://fb.me/react-derived-state
Please update the following components: AutosizeInput, Select

• Check anything else you can think of :)
• run yarn test to see that all the tests are passing (currently there are two console.error calls in another test that passes anyway, so ignore them)
• Beware that the styling has yet to be fixed (the button goes below the select, and if you use the searching in the select there is a blueish border on focus of the internal text input)

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes [P3-144]

[JesserH]

CR ✅

[JesserH]

Acceptance ✅