Comments created with one theme aren't displayed with another

[tcitworld]

Issue details

Because of the different structure template, the XPath paths are different : for a same paragraph on material : /div[1]/main[1]/div[1]/div[1]/article[1]/p[2] and baggy : /p[5].

We should modify the baggy or material themes in order to have the same structure to avoid this.

EDIT : Moreover, the path should consider only the content if we want comments to be retrieved through the API and displayed correctly.

Steps to reproduce/test case

Use baggy (or material)
Create comments. They are properly displayed.
Change theme to material (or baggy). Access the entry again.
The comments created before aren't shown.

See #1653