Support for memo-based pooled accounts

[JakeUrban]

Background

Pooled accounts are Stellar accounts that hold funds for multiple users. To disambiguate users who use pooled accounts, we're moving forward with a memo-based approach, where a given user is identified by the shared Stellar account AND a memo (or user id). We are currently updating our SEPs to integrate this mechanism, and would like the demo wallet to support users with this type of account.

You can see the changes being made to the protocols (10, 24, 31, 6, 12) here: https://github.com/stellar/stellar-protocol/pull/1036

Changes

Generally, we need to give users of the demo wallet the ability to provide an account ID or memo after adding a Stellar account. This memo should then be used in accordance with the SEPs in all interactions with anchor applications.

UI:

• allow users to enter a memo to be used after or while adding a Stellar account
• allow users to modify the memo used

SEP-10:

• passing the memo and type in the GET WEB_AUTH_ENDPOINT call

SEP-24:

• allow users to include the memo in POST /transactions/interactive/deposit requests, or provide a custom memo
  • Applications using custodial accounts may provide per-account or per-transaction memos to use when sending funds

SEP-6:

• same as SEP-24

SEP-31:

• If a memo has been entered, confirm that the memo should be associated with the sending user of the transaction
  • Since the Stellar account in a SEP-31 transaction belongs to the sending anchor, the memo ID must be for the sending customer, otherwise it doesn't make sense
• pass the memo and type when calling GET /customer and PUT /customer for the sending customer
• while we're at it, we might as well allow users to provide a memo to use for the receiving customer, but this should not be saved outside of the SEP-31 flow

[JakeUrban]

@stfung77 do you want to assign this issue to yourself if you're taking this on?