Recommended fixes and signup flow improvements

### What's new

Some minor changes:
- changed module category to `advertising_marketing`
- updated module description
- sign up page tweaks
- use of fancybox for screenshots

The bulk of changes in this PR are to support a streamlined onboarding procedure for new stores. With these changes we can now:
- automatically fill in store information for their new LoyaltyLion account (e.g. store name, language, currency, PS version)
- insert their LoyaltyLion token & secret into their PrestaShop store with one click
- create hundreds of voucher codes in PrestaShop with one click

The latter two methods work by opening up a pop-up window to their LoyaltyLion module configuration page, passing it custom parameters like `ll_set_token_secret` which the module now picks up.

Have changed this function to explicitly return null -- the caller checks a currency exists so null is safe to return here.