Minor fixes for tagging django admin [FC-0036]

[bradenmacdonald]

This fixes a few minor issues I found while reviewing openedx/edx-platform#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

[openedx-webhooks]

Thanks for the pull request, @bradenmacdonald! Please note that it may take us up to several weeks or months to complete a review and merge your PR.

Feel free to add as much of the following information to the ticket as you can:

• supporting documentation
• Open edX discussion forum threads
• timeline information ("this must be merged by XX date", and why that is)
• partner information ("this is a course on edx.org")
• any other information that can help Product understand the context for the PR

All technical communication about the code itself will be done via the GitHub pull request interface. As a reminder, our process documentation is here.

Please let us know once your PR is ready for our review and all tests are green.

[pomegranited]

👍 Thank you for these fixes @bradenmacdonald !

• I tested this using Django Admin on my devstack.
• I read through the code
• I checked for accessibility issues N/A
• Includes documentation

[bradenmacdonald]

Thanks for this fix - looks good! 👍🏻

[openedx-webhooks]

@bradenmacdonald 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.