[Notifications] Update notifications page to new UI

[aniri]

New UI is available here: https://www.figma.com/file/61NHR7fwmV1Dsv4Pb6F8T7/code4ro_mv_user-management_2018.04.21-(Copy)?node-id=754%3A0

• [ ] update to new UI elements
• [ ] by default, notifications should be sent to all observers
• [ ] filtering should be done by county and polling station numbers (a range of numbers)
• [ ] the number of observers is shown live in the CTA button and in the filter section after applying filters
• [ ] the send notification call to action button text changes after filtering

Please use bootstrap classes as much as possible and avoid adding custom css styles to keep the codebase clean & consistent. The new UI should be as similar as possible to the one in figma, not pixel perfect.

Also, make sure all texts are extracted in translation files.

[aniri]

Here's a preview of how it should look like:

[nicolaes]

Starting work on this.