Claim base game for Add-On if free

[vogler]

Start: 04f503f - mention URL of base game:

Free games: [
  'https://store.epicgames.com/en-US/p/chess-ultra-91bba3',
  'https://store.epicgames.com/en-US/p/world-of-warships--starter-pack-ishizuchi'
]
Current free game: Chess Ultra
  Not in library yet! Click GET.
  Claimed successfully!
Current free game: World of Warships — Starter Pack: Ishizuchi
  Requires base game! Nothing to claim.
  Base game: https://store.epicgames.com/en-US/p/world-of-warships

Since it is not visible on https://store.epicgames.com/en-US/free-games if it is a full game or an Add-On, we need to load the page first, but then would need to claim the base game first and reload to continue.
-> Needs some refactoring, esp. pulling code out into functions.