WooCommerce Subscriptions 6.3.1

* Dev: Add 'woocommerce_subscription_is_manual' filter. PR#3102
* Tweak: Add action attributes to the add to cart form on subscription product pages. PR#3105
* Fix: Remove "Your subscription will be activated when payment clears" on the order received page when the subscription is already active. PR#3099
* Fix: [WP 5.0.2] Fix an issue where the Admin Subscriptions table was empty.

* Fix: Fix caching on data for the Upcoming Recurring Revenue and Subscriptions By Date reports. PR#3069.
* Fix: Fix compatibility with Disable Report Cache Updates mini-plugin. PR#3069.
* Fix: Make Stripe source ID unique while setting through REST API. PR#3090.
* Fix: Display synced variation product's next payment date after the variation is selected by a customer on the product page. PR#3094.
* Fix: Activate subscription after processing an IPN on parent orders previously processed by PDT. PR#3097.
* Tweak: Update incomplete retry migration status label styling and wording to "In-Progress" to avoid confusing in-progress migrations as being a problem. PR#3088

* Fix: Prevent fatal errors when the retry migration doesn't receive a valid retry object. PR#3042
* Fix: Fix PHP notices in subscription events by date report. PR#3078
* Fix: Don't trigger payment gateways while processing retries on staging sites. PR#2870
* Fix: Apply coupons and discounts to initial payment and early renewal carts. PR#3048
* Fix: Use the correct `for` attribute in the variable_subscription_trial_period label element. PR#3081
* Fix: [PayPal Standard] Prevent creating multiple orders to record a single sign up transaction that was previously handled by a PDT request. PR#3074
* Tweak: Remove the nonce from early renewal urls. PR#3085