Consolidate to single app

# TL;DR
Consolidate the various app directories - accounts, api, exp, studies, web, project - into a single app.

# Narrative
As a developer, I want the organization of my app code to make sense. Furthermore, I don't want a complicated dependency graph for migrations.

The choice to split lookit-api into several applications was ill-advised. Given the high cohesion/interconnectedness of the data model, it has created more problems than solved by imposing awkward conceptual boundaries that have become a nuisance when adding new features ("does this go in `accounts` because it deals with accounts? Or does it go in `web` because it's part of the "web" interface? What does "web" even mean - isn't the entire application, a web application?).

We should consolidate to a single app, and in the process blow away the previous migrations and create a new initial migration. This could even coincide with our new semver'd v1 (#558), to formalize the notion that this is the real v1 data model.

# Acceptance Criteria
- [ ] All old tests pass.

# Implementation Notes
## Ancillary (beneficial) effect on tests
This will also speed up tests (see #564 ), since we won't be forcing our test harness to dutifully recapitulate the entire evolution of Lookit's data model every time we run `python manage.py test`.

## New Directory Structure
The new directory structure should look (very roughly) as such:
```
.
├── config
│   └── settings
├── lookit
│   ├── admin
│   ├── backends
│   ├── fields
│   ├── forms
│   ├── media
│   ├── migrations
│   ├── mixins
│   ├── models
│   ├── queries
│   ├── scripts
│   ├── serializers
│   ├── static
│   ├── templates
│   ├── templatetags
│   ├── tests
│   ├── urls
│   └── views
└── requirements
```

(files not included - classes and functions from different files may be reorganized to account for the fact that "shared" code is now subsumed into a single app)

After the multi-phase move to this consolidated structure, we should have #558 ready to go for a v1 release that deletes the rest of the cruft.

# Implementation Plan
## Phase 0: New Directory Structure
App skeleton changes
- [ ] Create new `lookit` app
- [ ] Rename `project` to `config`
## Phase 1: Model Migrations
- [ ] `studies` --> `lookit`
- [ ] `accounts` --> `lookit`
## Phase 2: Stateless Code Re-org
Includes static, templates, view code, urlconfs, etc.
- [ ] `studies` --> `lookit`
- [ ] `accounts` --> `lookit`
- [ ] `web` --> `lookit`
- [ ] `api` --> `lookit`
- [ ] `exp` --> `lookit`
## Phase 3: Cleanup & Finalize
- [ ] Delete old app shells
- [ ] Cleanup migrations (reset `id_seq` and clean out old content types)