Cashtab supports tx requests from webapps by parsing tx information from URL params.
Currently only `address` and `value` are supported. PayButton and cashtab-components use this approach.
Ultimately, should migrate to only supporting bip21, as this can set address, amount, and optionally much more. However we should maintain support for the legacy approach until it is not required by PayButton or cashtab-components.
The various cases to handle here are unfortunately somewhat complex (bip21 or legacy? if legacy, do you have only value and not address? is legacy valid? are legacy params duplicated?). However, automated integration tests for how specific inputs are rendered are straightforward to add for this feature.
Will be able to simplify once users have migrated to the bip21 approach.