added non-json response to actionSendActivationEmail

[i-just]

Description

If for whatever reason activation emails won't send successfully (e.g. a misconfigured Email setting), any selected checkboxes on the Permissions tab are cleared, which can cause issues if the user is then saved without reloading the page or reselecting the desired permissions.

Return asSuccess/asFailure if the request accepts json; otherwise, redirect back to prevent the above from happening.

Related issues

#13061

[brandonkelly]

Source of the bug was that the templates were just checking if it was a post request, rather than specifically a post request to the users/save-user action.