fix logic error with search filter.

if a post was in more than one different level, it would be hidden from search from ALL levels because the original query would ALWAYS return its ID since it's page_id would exist for a level other that the current user's level.

this update adds a second query to find the posts that ARE in the current users level and subtracts that from the original query.
Fixed bug with choosing levels to hide ads from on advanced settings tab. (Thanks, Alain Fradette)
* Fixed bug where PayPal Express was adding tax twice for subscription charges. (This does not fix existing subscriptions on the PayPal side.)
* Fixed bug with the Stripe Webhook where non-PMPro orders were being added to PMPro via the webhook. If the customer_id cannot be found (i.e. it's a non-PMPro customer), the order is ignored. (Thanks, Jacob Glenn)
* The getMemberOrderByPaymentTransactionID() function has been updated to return false if no payment_transaction_id is passed in (instead of finding the first order where the id is blank). This is inline with the getMemberOrderBySubscriptionTransactionID() function.
Fixed bug where user_id = '' was breaking on some MySQL setups and keeping the pmpro_membership_orders table from being populated.
More fixes related to last commit RE inserting/updating orders on some MySQL setups.
Added en_GB translations. These change "billing state" to "billing
county". UK WordPress just need to set their WordPress lang to en_GB.

* languages/pmpro-en_GB.mo:
Updated version number.
Merge pull request #84 from alexbiddle/en_GB_i18n

Added en_GB translations.
Merge pull request #75 from normanyung/master

fix logic error with search filter.
GitHub contributions.
* Fix to code that hides posts from search to NOT hide posts that a user has access to but might also be accessible by users of a different level. (Thanks, normanyung on GitHub)
* Added a pmpro-en_GB translation that changes "State" to "County" on the billing address fields. (Thanks, alexbiddle on GitHub)