EDIT changelog, elaborate on v1.3 celery env vars

Give more detail on why Celery's environment vars were changed.

Fix defaults. Celery worker memory control options are not used by
default. The standalone version executes locally and the docker-compose
version does not use them.

"Download All" is now limited to just student answers only.

CHANGELOG.md

@@ -1,25 +1,30 @@
 # v1.3
-## Notable changes
-
-* Upgrades: There is a high risk of regressions due to many upgrades to the libraries used by ComPAIR.
 
+## Notable changes
+* Many dependencies, both frontend and backend, were updated.
 
 ### New Features
-* 1. "Download All" attachments button was added to generate and download a zip file of all submitted attachments in answers
-* 2. The "Assignment End-Date" feature was added for admin users to query for the assignments end-date. 
-
+* "Download All" attachments button was added to generate and download a zip file of all student submitted answer attachments. This can be found in an assignment's "Participation" tab under the "Attachments" column.
+* The "Assignment End-Date" feature was added for admin users to query for the assignments end-date. 
 
-### Environment Variable Changes
-* CELERY_ALWAYS_EAGER is now CELERY_TASK_ALWAYS_EAGER
-  * Default: false
-
-### New Environment Variables: For controlling memory leak growth in Kubernetes
+### New Environment Variables: For controlling worker memory leak
 * CELERY_WORKER_MAX_TASKS_PER_CHILD - Kills a worker process and forks a new one when it has executed the given number of tasks. 
-  * Default to 20
 
 * CELERY_WORKER_MAX_MEMORY_PER_CHILD - Set to memory in kilobytes. Kills a worker process and forks a new one when it hits the given memory usage, the currently executing task will be allowed to complete before being killed. 
-  * Default to 600MB
 
+## Breaking Changes
+Celery 4 introduced a new all lowercase environment variables system. ComPAIR
+is now using this new system. To adapt a Celery environment variable to
+ComPAIR, convert the original Celery variable to all uppercase and prefix it
+"CELERY\_". ComPAIR will strip the prefix and lowercase the variable before
+passing it to Celery. A few Celery environment variables were renamed in the
+new system, the ones supported in ComPAIR are:
+
+* CELERY_ALWAYS_EAGER is now CELERY_TASK_ALWAYS_EAGER
+  * Set to true if running stock standalone, see `compair/settings.py`.
+  * Set to false if running via repo's docker-compose.yml
+* BROKER_TRANSPORT_OPTIONS  is now CELERY_BROKER_TRANSPORT_OPTIONS
+* CELERYBEAT_SCHEDULE is now CELERY_BEAT_SCHEDULE
 
 # v1.2.12