Super user invites (#483)

This PR brings user invites logic to the Management service
via HTTP API. 
The POST /users/ API endpoint creates a new user in the Idp
and then in the local storage. 
Once the invited user signs ups, the account invitation is redeemed.
There are a few limitations.
This works only with an enabled IdP manager.
Users that already have a registered account can't be invited.

go.mod

@@ -32,7 +32,7 @@ require (
 	github.com/c-robinson/iplib v1.0.3
 	github.com/coreos/go-iptables v0.6.0
 	github.com/creack/pty v1.1.18
-	github.com/eko/gocache/v2 v2.3.1
+	github.com/eko/gocache/v3 v3.1.1
 	github.com/getlantern/systray v1.2.1
 	github.com/gliderlabs/ssh v0.3.4
 	github.com/google/nftables v0.0.0-20220808154552-2eca00135732
@@ -41,7 +41,7 @@ require (
 	github.com/patrickmn/go-cache v2.1.0+incompatible
 	github.com/rs/xid v1.3.0
 	github.com/skratchdot/open-golang v0.0.0-20200116055534-eef842397966
-	github.com/stretchr/testify v1.7.1
+	github.com/stretchr/testify v1.8.0
 	golang.org/x/net v0.0.0-20220630215102-69896b714898
 	golang.org/x/term v0.0.0-20220526004731-065cf7ba2467
 )
@@ -99,6 +99,7 @@ require (
 	github.com/srwiley/rasterx v0.0.0-20200120212402-85cb7272f5e9 // indirect
 	github.com/vishvananda/netns v0.0.0-20191106174202-0a2b9b5464df // indirect
 	github.com/yuin/goldmark v1.4.1 // indirect
+	golang.org/x/exp v0.0.0-20220518171630-0b5c67f07fdf // indirect
 	golang.org/x/image v0.0.0-20200430140353-33d19683fad8 // indirect
 	golang.org/x/mod v0.6.0-dev.0.20220106191415-9b9b3d81d5e3 // indirect
 	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c // indirect
@@ -112,7 +113,7 @@ require (
 	gopkg.in/tomb.v1 v1.0.0-20141024135613-dd632973f1e7 // indirect
 	gopkg.in/tomb.v2 v2.0.0-20161208151619-d5d1b5820637 // indirect
 	gopkg.in/yaml.v2 v2.4.0 // indirect
-	gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 	honnef.co/go/tools v0.2.2 // indirect
 	k8s.io/apimachinery v0.23.5 // indirect
 )