No revision

This PR is not meant to be merged yet. I simply want to start the discussion on a few points

I haven't done any refactoring yet. I would like to do that in another commit once this PR has been discussed and approved

Especially concerning the `generate_url` method in post. Why not simply use the uniqueness feature of the database? The likelyhood, that someone creates a post with the same title is low. Instead of asking the user to generate a meaningfull url in that case, we create a url that is indeed unique, but is poor in term of SEO search. I believe that a simple message in that case might simply be better

Also concerning the migration:
Should not we simply make the operation irreversible?`ActiveRecord::IrreversibleMigration`

I am going to add a few comments on lines that need discussion.
Cheers,
M