Base meta-analysis workflow interface 

@ElDeveloper and I have added a basic implementation of the:

- login screen
- the analysis creation page
- study selection page

to test this, from the base repository cd to qiita_pet and run webserver.py

```
cd qiita_pet
python webserver.py
```

You should be able to go to a browser and enter `localhost:8888` to see the main page being served.

NOTE: login won't work since the database is not set up yet.

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We've also added a method to Study to retrieve all the currently public studies in the database.