Restrict editor programs for mapping and validation stages

>**This PR includes a db migration**

Related Issue: hotosm#893

* Added list of editors in the `Settings` menu of a project-edit page where a project manager can select permitted editors for each editing stage
![Screen Shot 2019-03-21 at 10 37 50 AM](https://user-images.githubusercontent.com/9849118/54944416-10f7a880-4ef9-11e9-8824-6867463ebf3c.png)

* Updated the editor dropdown for to populate only with the permitted editors
![Screen Shot 2019-03-25 at 12 28 35 PM](https://user-images.githubusercontent.com/9849118/54944636-90857780-4ef9-11e9-9797-c8bd7a727e72.png)

Dockerfile

@@ -14,6 +14,8 @@ WORKDIR /src
 
 WORKDIR /src
 
+WORKDIR /src
+
 # Add and install Python modules
 ADD requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt

docker-compose.yml

@@ -9,6 +9,8 @@ services:
     env_file: tasking-manager.env
     depends_on:
       - postgresql
+    expose:
+      - "5000"
     ports:
       - "5000:5000"
     links:
@@ -41,7 +43,6 @@ services:
       driver: none
     volumes:
       - pgdata:/var/lib/postgresql/data
-
 volumes:
   pgdata:
   stage: