[bug 949101] Add mlt section to response view

This adds a morelikethis section to the response view. It's "prototype"
quality. I had a hard time fine-tuning the MLT query and ended up just
adding a massive stopwords list which seemed to reduce the number of
"wow--that has nothing to do with this" responses. This might benefit
from synonyms if we ever add that.

This also adds a test for the response view to make sure the "secret"
section only shows up for analyzers.
Cosmetic: change to imports
Cosmetic: Remove dead code
Fix bigrams computation

This switches computing bigrams to use the standard analyzer rather than
the snowball analyzer because then we don't get stemmed weirdness.
Simplify and speed up indexing

This removes some code from indexing that we don't really need and
allows us to switch to a list comprehension which speeds things up.
[bug 1007826] Remove dashboard link

The dashboard link leads users to a meh dashboard that's confusing at
best and easily misinterpreted. Amongst other things, the link reads
"Want to see where your feedback goes?" which suggests the dashboard is
a destination for feedback when it's just a (lousy) view of the
feedback. It doesn't talk about any of the systems we use to aggregate
and examine the feedback.

This removes the link for now. When the new dashboard system lands, we
can reinstate it.

This also removes some dead code which never executes since "opinion"
is never defined.
[bug 1007833] Add note about version filter

The version filter facet didn't show up at all unless you picked a
product first (need a product in order for versions to make sense).
However, that wasn't clear.

This adds a note which should make that clearer.