Minor UI improvements

[pombredanne]

• do not use PK in URLS as the 36 in http://127.0.0.1:8001/vulnerabilities/36?vulnerability_id=VULCOID-10
• use a neutral "search" form field name and URL field name for search rather than vulnerability_id and package_name
• remove the search terms repeated in the vulnerabilities and packages views (actually only packages)

[pombredanne]

Completed in #909