[DEV-1954] Add settings for user-facing name of tags.
[DEV-1954] Change verbose_name and verbose_name_plural of Tag to match settings.
[DEV-1954] Update search to take [keyword:] instead of [tag:].
[DEV-1954] Change use of "tag" in templates to draw from settings.
[DEV-1954] Replace uses of 'tag' with setting in Python code.
[DEV-1954] Wrap settings access in const in functions to prevent circular import.
[DEV-1954] Avoid revolting circular import by hard-coding 'keyword' in const.
[DEV-1954] Fix variable assignment in trans tag.
[DEV-1954] Update tag tests to use new setting.
Merge pull request #34 from roverdotcom/DEV-1954

[DEV-1954] Change 'tags' to 'keywords.'