Implements the new `Recent cases` widget on the Overview page.

Recent cases shows the last 3 recently created cases, per the following animated gif:

![recent-cases](https://user-images.githubusercontent.com/4459398/77357982-ae550a80-6d0e-11ea-90d0-62fa5407eea5.gif)

Markdown case descriptions are rendered, per the following animated gif:

![markdown-description](https://user-images.githubusercontent.com/4459398/77358163-f7a55a00-6d0e-11ea-8b85-dd4b3ff093ee.gif)

My recently reported cases filters the widget to show only cases created by the logged-in user, per the following animated gif:

![my-recent-cases](https://user-images.githubusercontent.com/4459398/77358223-14419200-6d0f-11ea-8e4a-25cd55fdfc44.gif)

A message welcoming the user to create a case is displayed when no cases exist, per the following screenshot:

![no-cases-created](https://user-images.githubusercontent.com/4459398/77358338-4ce16b80-6d0f-11ea-98d3-5de1be19a935.png)

- [x] Case-related links were updated to ensure URL state parameters, e.g. global date selection, carry-over as the user navigates through case views
- [x] Recent timelines was updated to only show the last 3 recent timelines (down from 5)
- [x] All sidebar widgets have slightly more compact spacing

Tested in:
* Chrome `80.0.3987.149`
* Firefox `74.0`
* Safari `13.0.5`