utils: move repo_name_slug to utils2 to prevent import cycle on setup_db

After commit 57a733313e4f, 'gearbox setup-db -c my.ini' fails with an import
cycle as follows:

Traceback (most recent call last):
File "/home/tdescham/repo/contrib/kallithea/venv/kallithea-release/bin/gearbox", line 11, in <module>
sys.exit(main())
File "/home/tdescham/repo/contrib/kallithea/venv/kallithea-release/lib/python2.7/site-packages/gearbox/main.py", line 199, in main
return gearbox.run(args)
File "/home/tdescham/repo/contrib/kallithea/venv/kallithea-release/lib/python2.7/site-packages/gearbox/main.py", line 145, in run
return self._run_subcommand(remainder)
File "/home/tdescham/repo/contrib/kallithea/venv/kallithea-release/lib/python2.7/site-packages/gearbox/main.py", line 149, in _run_subcommand
subcommand = self.command_manager.find_command(argv)
File "/home/tdescham/repo/contrib/kallithea/venv/kallithea-release/lib/python2.7/site-packages/gearbox/commandmanager.py", line 78, in find_command
cmd_factory = cmd_ep.resolve()
File "/home/tdescham/repo/contrib/kallithea/venv/kallithea-release/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2324, in resolve
module = __import__(self.module_name, fromlist=['__name__'], level=0)
File "/home/tdescham/repo/contrib/kallithea/kallithea-release/kallithea/lib/paster_commands/setup_db.py", line 27, in <module>
from kallithea.lib.db_manage import DbManage
File "/home/tdescham/repo/contrib/kallithea/kallithea-release/kallithea/lib/db_manage.py", line 47, in <module>
from kallithea.model.repo_group import RepoGroupModel
File "/home/tdescham/repo/contrib/kallithea/kallithea-release/kallithea/model/repo_group.py", line 35, in <module>
import kallithea.lib.utils
File "/home/tdescham/repo/contrib/kallithea/kallithea-release/kallithea/lib/utils.py", line 48, in <module>
from kallithea.model.repo_group import RepoGroupModel
ImportError: cannot import name RepoGroupModel


i.e. kallithea.model.repo_group wants to import kallithea.lib.utils which
in turn wants to import kallithea.model.repo_group.

In fact there exists kallithea.lib.utils and kallithea.lib.utils2.
The current split is that 'utils2' contains 'simple' utilities, none of which
depend on kallithea models, controllers, ... In contrast, 'utils' does rely
on such kallithea classes.

As kallithea.model.repo_group was only include kallithea.lib.utils for its
repo_name_slug method, which has no dependency on other kallithea classes,
move that method (and its dependent recursive_replace) to
kallithea.lib.utils2 instead. This fixes the import cycle.