Export user tags as a list

[michaelklishin]

It is currently exported as a comma-separated list. It would be significantly more convenient to export it as a JSON array. We already do this for virtual host tags, for example.

In order to make this reasonably backwards-compatible we could introduce a new field and/or support both kinds of values for import.

[michaelklishin]

This change would make definitions exported on a newer version fail to import on the older releases. The opposite is not true, older definitions will be imported just fine with a trivial amount of effort. Perhaps this should go into 3.9.0. HTTP API client libraries will need some updating to update both types as well.