System/ContentPacks: Installed packs show 'install' action, cannot be uninstalled/deleted

## Expected Behavior

Installed content packs should show the `uninstall` action. `Delete` doesn't work since they are installed.

## Current Behavior

Installed content packs are highlighted as `installed`. The action is `install` not `uninstall` or similar.
`Delete` as action fails since the content pack still is installed.

## Possible Solution

Toggle the action based on the CP state.

## Steps to Reproduce (for bugs)

1. Create a dashboard
2. Add it to a content pack for later provisioning, ensure that the cp is installed
3. Show the content pack listing
4. Try to uninstall the installed content pack

## Context

Creating a dashboard within a content pack for later Vagrant provisioning.

## Your Environment

* Graylog Version: 3.1.3
* Elasticsearch Version: 6.8.5
* MongoDB Version: 4.0.0
* Operating System: CentOS 7
* Browser version: 78.0.3904.108