make old D2 categories entity use new attributes entity in association. ...

...This works because ALL category attributes are migrated upon upgrade and new ones (using D2) will be created in the new table. also improve documentation and changelog. closes #1255

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | #1255
| Refs tickets  | 
| License       | MIT
| Doc PR        | na