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.sum

@@ -398,6 +398,8 @@ github.com/rogpeppe/fastuuid v1.2.0/go.mod h1:jVj6XXZzXRy/MSR5jhDC/2q6DgLz+nrA6L
 github.com/rogpeppe/go-internal v1.3.0/go.mod h1:M8bDsm7K2OlrFYOpmOWEs/qY81heoFRclV5y23lUDJ4=
 github.com/rs/cors v1.8.0 h1:P2KMzcFwrPoSjkF1WLRPsp3UMLyql8L4v9hQpVeK5so=
 github.com/rs/cors v1.8.0/go.mod h1:EBwu+T5AvHOcXwvZIkQFjUN6s8Czyqw12GL/Y0tUyRM=
+github.com/rs/xid v1.3.0 h1:6NjYksEUlhurdVehpc7S7dk6DAmcKv8V9gG0FsVN2U4=
+github.com/rs/xid v1.3.0/go.mod h1:trrq9SKmegXys3aeAKXMUTdJsYXVwGY3RLcfgqegfbg=
 github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
 github.com/ryanuber/columnize v0.0.0-20160712163229-9b3edd62028f/go.mod h1:sm1tb6uqfes/u+d4ooFouqFdy9/2g9QGwK3SQygK0Ts=
 github.com/sagikazarmark/crypt v0.3.0/go.mod h1:uD/D+6UF4SrIR1uGEv7bBNkNqLGqUr43MRiaGWX1Nig=