Remove old promoted links code

This is replaced by the new best-bets implementation.  Best bets for
"jobs" and "visa" are populated on production in the new system, and we
want to retire the promoted link for "budget 2014", so the new
implementation covers all the result adjustments made by this code.

This implements https://www.pivotaltracker.com/story/show/71295084
Merge pull request #237 from alphagov/remove-promoted-links

Remove old promoted links code

https://www.pivotaltracker.com/story/show/71295084
Remove old healthcheck code

Remove support for scraping HTML results pages, and for accessing the
"combined" search endpoint.
Merge pull request #238 from alphagov/remove-old-healthcheck-code

Remove old healthcheck code
Add disable_popularity debug parameter

Adding debug=disable_popularity disables the use of popularity
information when searching.  This is useful for checking the effect of
popularity information on searches.  This can be combined with the
existing disable_best_bets by joining the options with a comma
separator.
Whitespace fix
Use .to_s instead of string interpolation

This is a bit clearer in intent.
Use refute_match instead of assert !~
Merge pull request #239 from alphagov/add_disable_popularity_debug_option

Add disable_popularity debug parameter
Don't import html_search_client; it's been removed

This is needed to un-break the health-check.
Merge pull request #240 from alphagov/health_check_fix

Don't import html_search_client; it's been removed