implement /packages/{name} as its own route

A few things added with this commit:

- aurweb.packages.util
    - A module providing package and pkgbase helpers.
- aurweb.template.register_filter
    - A decorator that can be used to register a filter:
      @register_filter("some_filter") def f(): pass

Additionally, template partials have been split off a bit
differently. Changes:

- /packages/{name} is defined in packages/show.html.
- partials/packages/package_actions.html is now
  partials/packages/actions.html.
- partials/packages/details.html has been added.
- partials/packages/comments.html has been added.
- partials/packages/comment.html has been added.
- models.dependency_type additions: name and id constants.
- models.relation_type additions: name and id constants.
- models.official_provider additions: base official url constant.

Signed-off-by: Kevin Morris <kevr@0cost.org>

test/test_templates.py

@@ -0,0 +1,15 @@
+import pytest
+
+from aurweb.templates import register_filter
+
+
+@register_filter("func")
+def func(): pass
+
+
+def test_register_filter_exists_key_error():
+    """ Most instances of register_filter are tested through module
+    imports or template renders, so we only test failures here. """
+    with pytest.raises(KeyError):
+        @register_filter("func")
+        def some_func(): pass