Add a loading state to the ChecTable component

[ScopeyNZ]

It's becoming pretty common-place for us to add a row with a loading indicator to a table, and define some stying for it. We should add this to the ChecTable component so we DRY it up:

https://github.com/chec/dashboard/blob/07b1ae185b3219b2c1ca8a90cca8e15e0af4849b/src/modules/settings/components/shipping/ZoneTable.vue#L145-L152

The only weird thing about this is that we'll need to tell the component how many columns are in the table.

[robbieaverill]

If only HTML supported something like colspan="*"