added _description to insta_feed_snippet

[markuskirch]

The missing _description constant of the newly created models insta.post and insta.profile create recurring errors in the logs.

This PR adds the missing tag and resolves the issue

[markuskirch]

@AjmalCybro We use your custom module in our Odoo environment, and it throws an error in our logs. _description of new models missing.

I fixed the issue, can you review this PR?

Thanks!

[markuskirch]

@CybroOdoo