Fix Brightcove blank account id warning

Change description

Fix bug where a default value for Brightcove account_id field (either default from field, or as set by SettingsMixin). Correct default will display in Studio edit, and get rid of warning on first save after entering a Brightcove URL (assuming an Account Id defined in settings).

Draft for now, will wait on merge of #35 and rebase.

Type of change

[x] Bug fix (fixes an issue)
[ ] New feature (adds functionality)

Checklists

Development

[ ] Lint rules pass locally
[ ] Application changes have been tested thoroughly
[ ] Automated tests covering modified code pass

Security

[x] Security impact of change has been considered
[ ] Code follows company security practices and guidelines

Code review

[x] Pull request has a descriptive title and context useful to a reviewer. Screenshots or screencasts are attached as necessary
[ ] "Ready for review" label attached and reviewers assigned
[ ] Changes have been reviewed by at least one other contributor
[x] Pull request linked to task tracker where applicable

appsembler / xblock-video

Fix Brightcove blank account id warning #37