Add a "Limit Orders" section to the WooCommerce System Status Report

[stevegrunwell]

This PR adds a "Limit Orders" section to the WooCommerce System Status Report (WooCommerce › Status), with helpful debug information:

This table will then also be parsed into the "Copy for support" version of the report:

### Limit Orders ###

Enabled: ✔
Limit: 100
Submitted orders: 0
Remaining orders: 100
Interval: weekly
Interval start: 2020-04-12T00:00:00+00:00
Interval resets: 2020-04-19T00:00:00+00:00

Fixes #7.