Remove tags field and django-taggit library

[suecarmol]

Description

Remove the tags field and everything that has to do with it.

Rationale

After #688 was merged, the tags field is no longer needed, therefore, the django-taggit dependency isn't needed either.

Phabricator Ticket

T282034

How Has This Been Tested?

Screenshots of your changes (if appropriate):

Types of changes

What types of changes does your code introduce? Add an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Minor change (fix a typo, add a translation tag, add section to README, etc.)

[jsnshrmn]

This looks great overall!
It initially seemed strange to me that you edited the initial migration, but I realized that you had to since the dependency is being removed.
Thanks for your work!