Create `ExternalPauri` model

[Dilraj Singh]

Description

Sometimes, Kavi Santokh SIngh directly embedded text from Gurbani or other granths in the middle of his poetry.

In other words, we need to save:

• The contents of the secondary text
• The footnotes that Bhai Vir SIngh wrote ABOUT the secondary text
• Where the secondary text should be embedded

I realize that it's almost always after a specific pauri... Therefore, we can do something like:

• pauri_id => Which pauri to add this text AFTER
• content
• bhai_vir_singh_footnote