flagging: Implement hook_theme and a template.

This is a issue that has fallen out from a discussion in #2456903: Investigate regenerating the module using Drupal Console →

In light of what we have for the flagging entity have versus what the console command outputs when scaffolding a new Content entity

This aspect of the flagging entity need to be refactored.

Here is a @socketwench 's quote from

https://www.drupal.org/node/2456903#comment-9762527 →

I'd be open to implementing hook_theme() and the template in a separate issue. We should be doing that anyways.