Don't assert method call expectations in tag tests

We don't need to care about whether or not a given method is called once
or twice or more, we just care whether the correct values make it into
the output.

Asserting that methods have been called a certain number of times should
be reserved for methods that have side-effects.
Add tag_web_url helper method

There is a difference in the intent of getting the web URL for a tag and
the web URL with all content for a tag. This commit just adds a new
method to allow the tag presenter to properly describe its intent when
getting URLs.
Rename with_tag_* methods to tagged_content_*

The names with_tag_url and with_tag_web_url are confusingly similar to
tag_url and tag_web_url. These names make the distinction between their
intent slightly clearer.
Make tagged_content_web_url call tag_web_url

It makes more sense for the URL to find content tagged with a given tag
to be defined in terms of the URL for the tag itself than the other way
around.

Also drop the `else` branch because it's unnecessary to explicitly
return `nil`.
Tidy up URLHelper tag tests
Provide a tag_web_url for organisation tags

Don't provide a tagged_content_web_url because there isn't currently a
sensible place to see all content tagged to an organisation.
Merge pull request #186 from alphagov/add-web-url-for-organisation-tags

Add web url for organisation tags https://trello.com/c/3aCHludy/115-display-organisation-metadata-2