meteor3: re-enable fixtures and sitemaps with async/Meteor-3 implementations

fixtures: drop @cleverbeagle/seeder (sync Mongo, unmaintained) for a small
idempotent async seeder (same dev accounts). sitemaps: drop dead
gadicohen:sitemaps for a WebApp.connectHandlers /sitemap.xml with the same
static page list (the per-fire section was already disabled). Verified on
dev boot: 6 users ensured, sitemap serving; REST smoke byte-identical.

.meteor/packages

@@ -49,7 +49,6 @@ aldeed:schema-deny
 dburles:collection-helpers
 reywood:publish-composite
 facts-base@1.0.2
-gadicohen:sitemaps
 nspangler:autoreconnect
 quave:synced-cron
 # lmachens:kadira

.meteor/versions

@@ -47,7 +47,6 @@ flowkey:raven@1.1.0
 fortawesome:fontawesome@4.7.0
 fourseven:scss@5.0.0
 gadicc:blaze-react-component@2.0.1
-gadicohen:sitemaps@0.0.17
 geojson-utils@1.0.12
 github-oauth@1.4.2
 google-oauth@1.4.5