Me: ES6ify the Profile Links components

[tyxla]

This PR ES6ifies the Profile Links components (profile-link, profile-links, profile-links-add-other and profile-links-add-wordpress):

• Replacing require with import
• Replacing module.exports with export default
• Replacing var with let or const
• Using method shorthand
• Using this instead of this.constructor
• Improving spacing and formatting here and there
• Fixing too long lines

These components are used in /me, in the Profile Links card.

Note: there are target="_blank" occurrences which are to be fixed in a separate PR: #7680.

To test:

• Get this branch going
• Go to /me
• Inspect the Profile Links card and verify there are no regressions in its functionality and appearance, including:
  • Verify adding a WordPress link works and stores it properly
  • Verify adding another URL works and stores it properly
  • Verify deleting a link works properly
  • Verify existing links are rendered properly
  • Verify the proper message is displayed when there are no profile links

/cc @roccotripaldi

Test live: https://calypso.live/?branch=update/es6ify-me-profile-link

[roccotripaldi]

👍 Nice clean-up!