MM-32133 shared channel username collisions (#17347)

Support for handling username collisions between remote clusters. Users belonging to remote clusters have their username changed to include the remote name e.g. wiggin becomes wiggin:mattermost.

@mentions are also modified so the munged username is replaced with the original username when the post is sync'd with the remote the user belongs to.

When adding remote users:
- append the remote name to the username with colon separator
- append the remote name to the email address with colon separator
- store the original username and email address in user props
- when resolving @mentions replace with the stored original username

model/user_test.go

@@ -211,26 +211,30 @@ func TestUserGetDisplayNameWithPrefix(t *testing.T) {
 	assert.Equal(t, user.GetDisplayNameWithPrefix(SHOW_NICKNAME_FULLNAME, "@"), "nickname", "Display name should be nickname")
 }
 
-var usernames = []struct {
-	value    string
-	expected bool
-}{
-	{"spin-punch", true},
-	{"sp", true},
-	{"s", true},
-	{"1spin-punch", true},
-	{"-spin-punch", true},
-	{".spin-punch", true},
-	{"Spin-punch", false},
-	{"spin punch-", false},
-	{"spin_punch", true},
-	{"spin", true},
-	{"PUNCH", false},
-	{"spin.punch", true},
-	{"spin'punch", false},
-	{"spin*punch", false},
-	{"all", false},
-	{"system", false},
+type usernamesTest struct {
+	value              string
+	expected           bool
+	expectedWhenRemote bool
+}
+
+var usernames = []usernamesTest{
+	{"spin-punch", true, true},
+	{"sp", true, true},
+	{"s", true, true},
+	{"1spin-punch", true, true},
+	{"-spin-punch", true, true},
+	{".spin-punch", true, true},
+	{"Spin-punch", false, false},
+	{"spin punch-", false, false},
+	{"spin_punch", true, true},
+	{"spin", true, true},
+	{"PUNCH", false, false},
+	{"spin.punch", true, true},
+	{"spin'punch", false, false},
+	{"spin*punch", false, false},
+	{"all", false, false},
+	{"system", false, false},
+	{"spin:punch", false, true},
 }
 
 func TestValidUsername(t *testing.T) {
@@ -239,6 +243,11 @@ func TestValidUsername(t *testing.T) {
 			t.Errorf("expect %v as %v", v.value, v.expected)
 		}
 	}
+	for _, v := range usernames {
+		if IsValidUsernameAllowRemote(v.value) != v.expectedWhenRemote {
+			t.Errorf("expect %v as %v", v.value, v.expectedWhenRemote)
+		}
+	}
 }
 
 func TestNormalizeUsername(t *testing.T) {