Add lastModified to a piece of Content

This PR adds the lastModified (in R2 and flexible-content) field to each piece of content and updates the unit test. 
It also refactor the representation of Tags in a content introducing Tagging. In that way tags are not replicated in memory for each content they appear. We also store in a Map the tagIds of tags that migrated section.

<b>How to Test It</b>
Unit tests should be successful and the result stored in csv should be the same. LastModified should be added to a content.