This adds `in_deletion_window` to the File, Release, and Project
models. The three models currently compose in deletion semantics
as follows:

1. A file is deletable iff it was uploaded within the last 7 days
2. A release is deletable iff all of its files are deletable
3. A project is deletable iff all of its releases are deletable

This results in no special casing: the deletability of
releases and projects is entirely "driven" by the deletability
of files, as files are the primary way in which users drive the
creation of releases and projects.

This PR does **not** connect the deletion semantics to actual
UI or view changes; I'll do that in a follow-up, to keep the
patches small.

Signed-off-by: William Woodruff <william@trailofbits.com>

Signed-off-by: Facundo Tuesca <facundo.tuesca@trailofbits.com>

Signed-off-by: Facundo Tuesca <facundo.tuesca@trailofbits.com>