[Cashtab] Better UI for param sends

Summary:
Now that PayButton is active in the wild, it's a more common user experience to scan a QR code with params.

• Stop showing a confusing alert about how only the amount param is supported
• Make sure that a confirmation modal comes up for any tx created by params, regardless of user setting

Test Plan:
npm test

npm start, go to Send screen, copy paste a valid param string i.e. ecash:qplkmuz3rx480u6vc4xgc0qxnza42p0e7vll6p90wr?amount=516.55, verify no alert, verify on "send" you get a confirmation modal

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14546