CLI: explicit 'paskia migrate' subcommand for legacy conversion

Serve never converts databases: with no paskia.kantadb it points at
'paskia init', or at 'paskia migrate' when legacy *.paskiadb candidates
exist. migrate converts a lone candidate, or the one named by --rp-id
when several exist; the rest stay in place. devserver fails fast with
the same hint. gitignore covers paskia.kantadb and *.converted-bak.

.gitignore

@@ -6,6 +6,8 @@ dist/
 package-lock.json
 paskia.sqlite
 *.paskiadb
+*.converted-bak
+*.kantadb
 *.data
 /paskia/frontend-build
 /paskia/_version.py