Display Permissions Granted by Role

Specific permissions granted by each Role should be listed in its Role Details UI.

## What?
Currently, the Role Details UI only describes what the Role grants in plain English:


While this may suffice in a majority of cases where admins want to grant permissions to users, there is no documentation to support the creation of custom roles where admins need a list of specific permissions to grant their role.

Listing what permissions each default Role grants will make it easier for admins to create custom Roles based on these default Roles because they can see what permissions they have to choose from.

Currently these permissions are only accessible via API, which is not very user friendly. And the only way to get the permissions of a specific Role is to know its specific alphanumeric `id`, which is an unreasonable expectation of users to know:

```
{
"id": "63e51980d0ffff4811e66c9e",
"name": "Alerts Manager",
"description": "Allows reading and writing all event definitions and event notifications (built-in)",
"permissions": [
"eventnotifications:edit",
"eventdefinitions:read",
"eventdefinitions:create",
"eventdefinitions:delete",
"eventdefinitions:execute",
"eventdefinitions:edit",
"eventnotifications:create",
"eventnotifications:delete",
"eventnotifications:read"
],
"read_only": true
}
```

## Proposal
I propose the list of permissions granted by a Role (including Custom Roles) be listed somewhere on the Role Details page, perhaps here:


## Your Environment

* Graylog Version: 6.1