Refine site search based on user feedback

[cpeel]

Refine the new site search experience after input from users. In brief, this:

• Removes the "jump to username" feature if the string is a username, now this needs to be prefixed with u:.
• Adds an icon to the search menu to get to the site search help page
• If no prefix was used the search page now lists suggestions to help the user (thanks @windymilla for this suggestion)
• Enables searching for a task by summary in addition to string (thanks @srjfoo for the suggestion)

Sandbox: https://www.pgdp.org/~cpeel/c.branch/refine-site-search/

Forum discussion: https://www.pgdp.net/phpBB3/viewtopic.php?t=81458