Revert "Added "availability" as a way of expressing "User X is available on date Y"

This reverts commit 25192f9e133152afab3f8d32f0f1b05fa2b2f170.
Fixed n:m map between user and spoodle_date with join table withou
explicit model. The n:m map between user and event still has an explicit
model, because I think an invitation can contain text (ie. own
attributes). Even "Availabilities" (user-date map) could contain
attributes (ie. priority etc.) but let's think about that later.