Define initial models and migrations

This task introduces 2 new models and migrations for decidim awesome:

vote_types: Adds a "grade" for each vote. A grade is just an integer that can be used for weighting later on. This is a one-to-one relationship with a proposal vote. A presentation class will use this information to show the results. The vote action will be tampered in order to introduce this gradation in this table. This also depends on the presentation class (that will be coded with different implementations in mind, using a manifest). Note that removing the awesome module will leave the standard decidim voting results and decidim won't be broken.
grades_cache: Will be used to cache all the sums from each grade, the count json field could containa a summary of each votes, in this example the "grade" 1 will have 12 votes:
```
{
  "1": 12,
  "2": 52,
  "3": 21
}
```

Green-Party-of-Canada-Members / gpc-decidim