Minimal fix for duplicate tags for null channel errors

[rtibbles]

Summary

Description of the change(s) you made

• Avoids using get_or_create as will error out when there are multiple ContentTag objects for the same tag_name and null channel combination
• Removes unused code in models.py that I discovered in the course of this
• Adds a test for syncing tags and a regression test for this fix
• Fixes a bug that would happen after copying a node due to not setting original_channel_id on a ContentNode during the copy operation on the frontend
• Fixes a bug that prevented any refresh of data from the backend if any changes were present in the changes table, even if they had been synced

Manual verification steps performed

1. Import a resource from one channel into another channel
2. In the source channel apply a tag "testtag"
3. In Django shell (./contentcuration/manage.py shell) create a tag with the same tag_name property

from contentcuration.models import ContentTag
ContentTag.objects.create(tag_name="testtag", channel_id=None)

4. In the channel that imported this resource, now initiate a sync, and be sure to select Tags to sync
5. See the sync is successful

For the copy bug, this replicates by doing the following:

1. Import a resource from another channel
2. Wait for the copy to complete
3. Edit the resource - in the failure case the edit modal will only partially render - with this fix it now renders properly.

Fixes #3690
Fixes other undocumented bugs.

[bjester]

LGTM! Will this fix #3526?

[pcenov]

@rtibbles none of the newly added fields such as Learning Activity, Level, Requirements and Category are being synced - is this intentional or an oversight?

[rtibbles]

This is an oversight - I'll need to make some updates to ensure these get synced.