SE-985 Bump completion to 1.0.3

Fixes authentication issue with mobile apps.

*JIRA tickets*: OSPR

*Discussions*: This change is already in master, see https://github.com/edx/edx-platform/pull/20039#discussion_r279601316

*Sandbox URL*: TBD - sandbox is being provisioned.

*Merge deadline*: "None"

*Testing instructions*:

See https://github.com/open-craft/edx-platform/pull/165 for test instructions.

The sandbox above has already been set up for completion, mobile, and with the demo course and users. The `staff` user is a superuser.

*Author notes and concerns*:

1. Note that unlike https://github.com/open-craft/edx-platform/pull/165, this change does not need to upgrade `edx-drf-extensions` and `xblock`, since they are already updated.
1. There's actually no reason why we couldn't use the latest `edx-completion==2.0.0`, since the [diff with `1.0.3` is minimal](https://github.com/edx/completion/compare/1.0.3...v2.0.0). However, we're matching current `master` here to be safe.

*Reviewers*

• [ ] @symbolist

• [ ] edX reviewer[s] TBD

*Settings*
```yaml
EDXAPP_FEATURES:
ENABLE_MOBILE_REST_API: true
```