Enhances the styling of the items table

[sagarbaver]

I've updated the styling of the table that lists all the items under a particular DynamoDB table page.

• Updates the first row to be a th instead of td and its background is now light grey in color.
• Makes the table responsive by using an appropriate Bootstrap class.
• Makes the "Actions" column (first column) sticky so that users can easily click on the "View" link while scrolling horizontally through a large row item.
• Every row has a style on hover that makes it easier to spot.

Note: The values in the above screenshot have been masked.

These styling changes make it easier to navigate rows with long string values. They improve the experience problems highlighted under #92.

[sagarbaver]

@rchl If you get a moment, could you please review this PR?

[rchl]

Thanks