Add external id to taxonomy tag list

[jesperhodge]

Context

Tags have an external_id field that's the identifier for syncing with external systems (may be defined externally).

We had an architectural discussion about tag identifiers and mutability, the outcome of which is this forthcoming ADR, which leads to this ticket.

Acceptance Criteria

• The tag list table has a new column external_id displaying this field
• This field is editable when creating a new tag.
• This field is immutable and not editable after initial creation of the tag.
• This field is searchable by the tag search in the table.

Open Questions

• As far as I know, the backend sets the default of external_id to the tag's value. (Need to verify that.) Should we make this field optional or not on create?
• If it's optional, how do we set it to its default value? Should there be a default value in the frontend? Or should the backend be the sole owner of the default value, and if so how do we display the field in the frontend? What's the UI?