Fix tag link on dag detail page when setting base_url with url prefix

[xianghuzhao]

This PR simply fix the tag link on dag detail page.
If use reverse proxy and set webserver.base_url with url prefix, these tag links will point to wrong locations.
Here just replace the direct link with url_for function.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

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[boring-cyborg]

Awesome work, congrats on your first merged pull request!