Added Issue Search

Refactored issue search so that instead of searching by issue_iid, from has a 'search_string' which makes a project.search() request on API. Created template, view, and URL path for search call and added link to search in user-landing and on sidebar. Added an 'extra_links' block to layout template that includes 'shared/extra_sidebar.html', allowing for population of user-specific links once user is 'logged in' to landing page.