Add ellipsis to truncated card summary.

The change looks good, but we cannot simply apply it as-is: we need to allow existing sites to opt-out using a feature flag (we do not want to expose this as a per-component option because the functionality here should be enabled by default).

Thanks Alex, I've updated the patch as per the changes.

Sorry, last patch was missing a few more changes.

I've added the constant and display the option in the theme settings available flags.

Verified and tested on 1.x-dev

Testing Results:
1. Verify there is ellipsis added to truncated card summary indicating that the text has been trimmed (more than 160 characters). - PASS
2. Verify manual and automated cards - PASS

Screenshot:

Moving to RTBC.

Automatically closed - issue fixed for 2 weeks with no activity.