Billing and settings fixes

A round of fixes on the agency side of the app.

Billing. The links from Digitalsero into the Stripe customer portal were landing some agencies on Stripe's "page not found". Change plan, cancel, update payment method and update billing information now open the right page every time and bring you back to Digitalsero when you are done. And if you had set your subscription to cancel at the end of the period, Renew now reactivates it immediately inside the app instead of sending you to Stripe.

Settings. The Company Logo upload in Look and Feel now shows the recommended size, so your logo is not letterboxed in the client header. Features your plan does not include, such as Agency Subdomain, Company Logo and Colour Theme, now carry a small badge with a tooltip explaining that it is a premium feature and that you need to upgrade to use it.