Refactor: support multiple users under the same account (#170)

* 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

go.mod

@@ -27,6 +27,8 @@ require (
 	gopkg.in/natefinch/lumberjack.v2 v2.0.0
 )
 
+require github.com/rs/xid v1.3.0
+
 require (
 	github.com/BurntSushi/toml v0.4.1 // indirect
 	github.com/fsnotify/fsnotify v1.5.1 // indirect