Seed database automatically

[saminouidri]

Find a way to seed the database automatically so as to enable the teachers to test the application by creating posts.

Right now, when creating a post, placeholders for space_id, user_id and tag_id are used. They are all set to 1, so the database must have entries with that id for each object.

[ylked]

fixed by allowing null values for space and tags fields, see #40