Minor fixes for tagging django admin [FC-0036]

[bradenmacdonald]

This fixes a few minor issues I found while reviewing https://github.com/openedx/edx-platform/pull/33563 :

• Some minor improvements to the django admin. I know we don't want to rely on the django admin too much, but it's sometimes handy to inspect what's going on with tagging/taxonomy model data.
• Explicitly remove "Add" button for ObjectTags and Tags.
• Editing a "normal" Taxonomy in the django admin was not possible, because _taxonomy_class didn't have blank=True. I fixed that.
• Some fields in the admin like ObjectTag.tag were loading all possible tags into a select widget, which could be hundreds of thousands. I changed it to an auto-complete widget.
• On my machine, the Language Taxonomy didn't have the right _taxonomy_class set. I think this is due to the messy migration history. I included a fix in case anyone else has the same issue on their machine. I was trying to figure out why the API was not showing "Languages" as system-defined.

ObjectTag before:

ObjectTag after:

Tag before:

Tag after:

Private-ref: FAL-3523

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