Filter for only the Blocked users

Here is a patch that fixes the problem with the filters going back to default state and also added the option to only filter on the blocked request.

Might need some refinement, but at least its a start.

I checked this:

- number of rows is saved as it should be, when navigating through various results pages

- labeling of the checkbox can be improved

From:

'#title' => $this->t('Filter blocked requests.'),

To:

'#title' => $this->t('Only blocked'),
'#description' => $this->t('Show only the blocked requests'),

We dont need to use the words "Filter" and "requests" because it is implied by the context. A small desription can clarify the functionality

Worked on #5. Please review.

Thanks everyone for their efforts.

Automatically closed - issue fixed for 2 weeks with no activity.