fix display of conversations search results.

fixes CNVS-4729

test plan:
  * as a user in a course, navigate to the inbox;
  * attempt to locate a recipient for a new message
    using the "To" field and verify that the results
    are spaced properly and that the active result (e.g.
    the one w/ the blue background) text is light enough
    to read;
  * search existing conversations using the "filter" text
    input and verify that its results also display
    properly.

Change-Id: I7564d860e8ff0c6989ea65dfd885a69f38b7b0f6
Reviewed-on: https://gerrit.instructure.com/18813
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Eric Berry <ericb@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>

app/stylesheets/utilities/_context_search.sass

@@ -38,6 +38,7 @@
       white-space: nowrap
       overflow: hidden
       display: block
+      line-height: 16px
     b
       color: #000
       text-shadow: none
@@ -69,7 +70,7 @@
       color: #fff
       text-shadow: 0px 1px 1px #0587bb
     span
-      color: #0d4276
+      color: #e1edf0
     a.toggle i
       background-position: 0 -48px
     img.user.avatar