Port table header styles from admin template

* Add table example to style guide
* Include styles for table header and secondary table headers
Apply ported table styles to stats listing

* Give the table a prominent header consistent with other admin apps
Port and apply no content styles from admin gem

* A large but light style which applies to the empty state.
* Apply when no statistics announcements are found by a filter
Cleanup stats announcement page

* Indicate statistics type above title, matching front-end
* Clarify the view on website link, specifying which site
* Remove the warning message and fix grid layout of page
* Format details and summary in the same style as used on the front-end
* Remove delete announcement button
Cleanup document linker

* Delete existing overrides and inline styles
* Update language from “linking” to “connecting” to avoid confusion
related to behaviour of legacy systems, as observed in user research
* Re-style document finder, which uses IDs for styles and javascript
which makes refactoring or overriding styles difficult