Moving some widget table logic to new `DashboardWidgetTable` class.

To reduce code duplication, especially regarding the common styling of
dashboard widget tables.
Moving additional CSS styles to dashboard.css.
Converting ThemeHelperWidgetTable to `DashboardWidgetTable`.

Also fixing styling to match the `DashboardWidgetTable`. Changed the name
at the top of the widget so that its purpose is apparent with the new
styles.
Added admin notice when a repo is out of date.

If one repo is out of date, it shows the repo status text and the name of
the repo. If more than one is out of date it shows the number of out of
date repos.

Once implemented this message should give a link to updated the out of
date repos.