This PR simplifies Store and FileStore
by keeping just the Get and Save account methods.
The AccountManager operates mostly around
a single account, so it makes sense to fetch
the whole account object from the store.
* feature: add User entity to Account
* test: new file store creation test
* test: add FileStore persist-restore tests
* test: add GetOrCreateAccountByUser Accountmanager test
* refactor: rename account manager users file
* refactor: use userId instead of accountId when handling Management HTTP API
* fix: new account creation for every request
* fix: golint
* chore: add account creator to Account Entity to identify who created the account.
* chore: use xid ID generator for account IDs
* fix: test failures
* test: check that CreatedBy is stored when account is stored
* chore: add account copy method
* test: remove test for non existent GetOrCreateAccount func
* chore: add accounts conversion function
* fix: golint
* refactor: simplify admin user creation
* refactor: move migration script to a separate package
