importer: ~120 laddr tags have no resolvable namespace

[themightychris]

About 12% of laddr's tags are skipped by the JSON importer because their
Handle is a bare word (cocoa, aws, geodjango, naloxone,
organizing_team, ...) with no topic. / tech. / event. namespace
prefix, and the Title field doesn't carry it either (in most cases the
Title is just the same bare word).

These were tags created via laddr's autocomplete-create flow without
typing a namespace, so the source data lacks the information we need to
classify them. The v1 Tag.namespace enum is topic | tech | event and
there's no catch-all.

Options:

1. Default to topic — assume anything namespace-less is a topic tag.
   Simple but pollutes the topic taxonomy.
2. Hand-classify — small effort (~120 tags) before cutover; produces
   the cleanest taxonomy.
3. Leave skipped — accept the loss. Most of these are low-traffic tags
   (org names, single-event keywords) that nobody currently uses for
   browsing on codeforphilly.org.

The importer logs [tags] legacyId=<n> handle "<x>" has no resolvable namespace; skipped for each skip, so the list is enumerable from any
--verbose run.

Surfaced during the laddr-import-via-json plan (PR #57).