[BUGFIX] Better detection of application models

Created by: gracewashere

Closes #122 (closed).

Problem:

Administrate assumes that all subclasses of ActiveRecord::Base are backed by database tables.

When a host application or gem defines a subclass of ActiveRecord::Base that doesn't have a corresponding database table, the installation generator sees an error:

postgresql_adapter.rb:596:in `async_exec': PG::UndefinedTable: ERROR:  relation "read_marks" does not exist (ActiveRecord::StatementInvalid)
LINE 5:                WHERE a.attrelid = '"read_marks"'::regclass
                                          ^
:               SELECT a.attname, format_type(a.atttypid, a.atttypmod),
                     pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
                FROM pg_attribute a LEFT JOIN pg_attrdef d
                  ON a.attrelid = d.adrelid AND a.attnum = d.adnum
               WHERE a.attrelid = '"read_marks"'::regclass
                 AND a.attnum > 0 AND NOT a.attisdropped
               ORDER BY a.attnum

Solution:

Verify that a model's database table exists before creating the model's dashboard.