Last saved date appears to be incorrect

[pwang347]

Application or Package Used
State which application or package this affects
nteract desktop (0.28.0)

Describe the bug
Last saved value in the content model is a Date type, but the value for it seems to be always set to a string.
See the following:

• https://github.com/nteract/nteract/blob/main/packages/reducers/src/core/entities/contents/index.ts#L133-L134
• https://github.com/nteract/nteract/blob/main/packages/reducers/src/core/entities/contents/index.ts#L277-L278
• https://github.com/nteract/nteract/blob/main/applications/desktop/__tests__/renderer/epics/loading.spec.ts#L67

Possibly related, on the desktop app the last saved status seems to never update.

To Reproduce
Steps to reproduce the behavior:

1. Open the nteract desktop application
2. Wait a few minutes
3. Observe that the last saved time doesn't update

Expected behavior
After waiting a few minutes, the message "Last saved less than a minute" should update.

Screenshots

Desktop (please complete the following information):

• OS: Windows 10