templatetags: add utils template filters and tags

templatetags: add utils template filters and tags

Add utils.py file to create template filters and tags that can be used
by most if not all objects in Patchwork. In particular, add a template
filter to get the plural verbose name of a model and add a template tag
that returns whether an object is editable by the current user. These
utilities will be used in an upcoming patch that adds the `addressed`
status label to patch and cover comments.

Signed-off-by: Raxel Gutierrez <raxel@google.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>
Signed-off-by: Daniel Axtens <dja@axtens.net>