global: new publication subtype `Proposal`

* Adds `Proposal` subtype to `Publications`.

Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>
@lnielsen-cern, what I found is that `upgrader` is sorting the list of latest upgrades in this way: 

```python
u = Upgrade.query.filter(Upgrade.upgrade.like("%s_%%" % repository)).order_by(desc(Upgrade.applied)).first()
```

So in my case (local environment) if the upgrades were all applied on the same date, there exists a scenario that the method will not return the latest (based on name) upgrade.
Instead of `zenodo_2014_03_18_schtask_fix` I got `zenodo_2014_02_04_old_tables`.