support untagged content

[cbuv]

What does this pull request change?

• remove tags in blogpost, tags should pass review.
• support untagged content (doc-themes should always have a tag, defaults to document icon)
• margins should be outside components (TagFilter)
• Icon's in DocListing is too big, constraint to 30x30px. ref sketches

Why is this pull request needed?

Should not be required to provide tags.

What should be tested by reviewer?

remove tags content folder and test pages.

Checklist

• [x] formatting

[eoaksnes]

When I select the tag "Api" in search page, nothing happens.

[cbuv]

When I select the tag "Api" in search page, nothing happens.

fixed in d581c5f. The filter tests assumed all posts was tagged. Added test for new behavior where posts can be untagged.