Add support for SEP-0007 pay and transaction requests

[ebma]

Supersedes #1179.

• Add transactionRequest context (to parse and store incoming SEP-0007 requests)
• Add <TransactionRequestHandler> that checks the uri stored in the transactionRequest context and shows dialogs according to its content
• Add <VerifyTrustedServiceDialog> that is shown for incoming SEP-0007 requests that have an origin_domain but are not in the list of trusted services. The user can then decide to trust this domain and it will be added to the list of trusted services.
• Add <PaymentRequestContent> that renders an inline payment form and account selection list
• Add <TransactionRequestContent> that renders a transaction summary and an account selection list, either for selecting the source account (if a source account -replacement is defined) or for selecting the signing account
• Add <StellarRequestReviewDialog> that will render the PaymentRequestContent or TransactionRequestContent based on the type of the incoming SEP-0007 request.
• Add preselectedParams as a prop to the <PaymentForm> to be able to pre-fill the respective input fields with values of the pay request. Pre-filled input fields will be disabled so that the user cannot change the value.
• Enable trusted services by default (Remove the feature flag)
• Save settings to localStorage in web build (makes it easier to test trusted services but is useful in general)
• Add <NoAccountsDialog> that is shown if no accounts are available for the network specified in the SEP-0007 request

Closes #323.

Sample SEP-0007 requests can be found here and here.

Payment request:

Transaction request:

No accounts:

[cblp]

Good work! We really need this in our DAO!