Allow categorization of achievements [#3341585]

Problem/Motivation

In the Drupal 7 version of this module, it was possible to group achievements into categories. This does not seem to possible in the Drupal 8/9/10 version.

Proposed resolution

- Add a taxonomy vocabulary "Achievement Categories"
- Let users (optionally) choose a taxonomy term from the "Achievement Categories" vocabulary when creating/editing an achievement

Data model changes

- Add a category field to the achievement entity type

Issue fork achievements-3341585

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3341585-allow-categorization-of changes, plain diff MR !5
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