Add a public field to env vars

In the first version of environment variables which I already merged, I only added 2 fields to env var: name and value. The problem with this is that all of these variables would have to be shown in the build log as:

```
export FOO=[secret]
export BAR=[secret]
```

It's easy to imagine that you would like to add all of the variables needed for one thing in the UI, for example: S3_BUCKET, S3_KEY, S3_SECRET_KEY and for clarity, it would be nice if these could be displayed as:

```
export S3_BUCKET=a-bucket
export S3_key=[secret]
export S3_SECRET_KEY=[secret]
```

which is a bit more readable than showing everything as "secret"

This PR adds a public key to `EnvVar` model, which is false by default. It will be used to determine if a value of a variable can be shown in the UI and in logs. It will still be encrypted in the database, to keep implementation simpler.