admin: Fix ID filters

Fix bugs in filters due to earlier (0.5.3-3) django-filter version.

The main issue was that django-filter could not handle inherited fields.

Fix apyrgio/synnefo#202
admin: Do not use subqueries between DBs

Django cannot handle the case when a query points to the model of one
database and a subquery points to the model of another database. To
resolve this, due the queries separately and feed the results back as in
a chain.

Fix apyrgio/synnefo#205
admin: Search with "startswith" for "snf-*"
Merge branch 'feature-admin-filters' into feature-admin