Create and cancel a subscription request

[luchobonatti]

User story / Problem statement

An allowlisted investor needs to subscribe, and to back out while the price does not exist yet.

Expected outcome

A subscription request escrows the deposit asset, and cancelling returns it.

Acceptance criteria

• The request is refused when the investor is not allowlisted, when requests are paused, and when a request is already open on either side
• Each refusal is shown before the investor signs, not after
• The escrowed amount is visible while pending
• Cancel returns the full amount in one transaction
• Cancel is offered while pending and gone once priced

Technical notes

Blocked by #65: no vault contract exists. Blocked by #85. Parent: #30.