Validate required fields on install content pack

## Expected Behavior
Installing a content pack provides feedback when a field is not fulfilled and does not reset the form in the process.

## Current Behavior
Installing a content pack without providing all required fields looks like the form is submitted, but it isn't. The modal closes and there is no clear indication of what happened. All inputs will be reset if the modal is opened again.


There are no errors in the browser's console or the server logs.

## Steps to Reproduce (for bugs)
1. Create a content pack with parameters, leave some default values empty, as they may be optional
2. Install the content pack and don't fill one of the inputs
3. Check that the content pack install was not submitted to the server and no error is displayed
4. Opening the modal again shows all default values again

## Your Environment
* Graylog Version: Graylog 3.0.0-rc.1-SNAPSHOT (4a8ae47)