[WIP] Fix S3 edxapp_media_dir leading slash issue

Description

(Note: This is a WIP PR that is currently being tested and I will update the status once the changes are confirmed to work.)

`django-storages` 1.8 throws an error when the storage location path for an S3 bucket starts with a leading `/`. (Reported in (https://openedx.atlassian.net/browse/CRI-206)). This happens because `edxapp_media_dir` used as the default value of the `location` parameter and it has a default value of `/edx/var/edxapp/media`. The older versions of `django-storages` automatically translated `/edx/var/edxapp/media` to the path `edx/var/edxapp/media` on the S3 buckets. But in 1.8 they raise an `ImproperlyConfigured` exception ([code](https://github.com/jschneier/django-storages/blob/1.8/storages/utils.py#L82)).

This PR attempts to fix this issue by removing the leading `/` from the variable passed to the `location` parameters.

Reference: [discussion on Slack](https://openedx.slack.com/archives/CSAC4QHC7/p1589971174063800)

CC @nedbat

Configuration Pull Request

Make sure that the following steps are done before merging:

  • [ ] A DevOps team member has approved the PR if it is code shared across multiple services and you don't own all of the services.

  • [ ] Are you adding any new default values that need to be overridden when this change goes live? If so:

  • [ ] Update the appropriate internal repo (be sure to update for all our environments)

  • [ ] If you are updating a secure value rather than an internal one, file a DEVOPS ticket with details.

  • [ ] Add an entry to the CHANGELOG.

  • [ ] If you are making a complicated change, have you performed the proper testing specified on the [Ops Ansible Testing Checklist](https://openedx.atlassian.net/wiki/display/EdxOps/Ops+Ansible+Testing+Checklist)? Adding a new variable does not require the full list (although testing on a sandbox is a great idea to ensure it links with your downstream code changes).

  • [ ] Think about how this change will affect Open edX operators. Have you updated the wiki page for the next Open edX release?

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Guruprasad Lakshmi Narayanan

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edx/configuration

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