Pallet for Purchase (#1369)

* initial mock

* remove statement

* Merge branch 'master' into shawntabrizi-crowdsale

* only work for dead accounts

* Revert "Merge branch 'master' into shawntabrizi-crowdsale"

This reverts commit 6f8fde1b

.

* update storage name

* Re-mock

* Update comment

* payouts

* test signature verification

* Update wording

* fix tests

* support both free and locked balance

* Single statement set in configuration trait

* Configurable purchase limit, initiated status

* Account creation tests

* make note about `max_amount` check

* Update validity status tests

* update balance tests

* payment test

* finish tests

* enable in runtime

* Test and verify ed25519

* make purchase pallet more configurable on the fly

* Remove runtime check on purchase amount.

* clean up trait

* add basic weights

* add extra super saftey check for vesting

* Add vat tracking

* remove unused const

* Update to W3F Origins

* remove stale comment

* Support existing accounts w/o existing vesting schedule

* Update runtime/common/src/purchase.rs

Co-authored-by: Gavin Wood <[email protected]>

* Add support for partially unlocked portion of purchased DOTs

* add trait import

* Expose constants from pallet

* Fixes

* Fixes

* Update runtime/polkadot/src/lib.rs

Co-authored-by: Gavin Wood <[email protected]>