Generalize the BudgetInvestmentStatus model to Milestone::Status so it
is not specific to budget investments, but can be used for any entity
which has milestones. This is in preparation to make the Milestone
model polymorphic and usable by entities other than budget investments.
Most factories were using dynamic times and dates since commit 0cf799a.
However:
* At the time, commits AyuntamientoMadrid/consul@71f5351 and
AyuntamientoMadrid/consul@a476a30 (which introduced static times/dates
in factories) hadn't been backported.
* The changes in commit 0cf799a overlooked the factory
`proposal_notification`.
