HTML-safe the data section of DataSet show page

This should address [#41917891], using the code we originally used when showing this data in PredefinedGraphPanes (see https://github.com/concord-consortium/smartgraphs-authoring/commit/3724a6bdcb6ee7103febf92fc031d671c0df1c01#L0L608 for the commit where I removed it).
Creates BestFitSequence model, with spec and migration

[#40210301]
Controller, helper, and view(s) for BestFitSequences

This makes the data model changes for [#40210301] available in the Hobo UI.
WIP on testing the BestFitSequence
created a spec for the SgMarshall module
Rails update to address vulnerability

ref: https://groups.google.com/forum/?fromgroups=#!topic/rubyonrails-security/61bkgvnSGTQ
Cucumber scenario for BestFitSequence

[#40210301] This passes up to copying the activity, but fails that last step.
Merge branch 'master' into line-of-best-fit
Updating generator to line_of_best_fit tag

[#40210301]
Created a rake task to load semantic data in a directory into a DB [40638719]
Making the dataset association work in Cucumber
added seed data for cucumber and changed DB cleanup strategy to transactional
Noting valid-activity requirements for BestFitSequences

Data sets of less than 6 points cause an error. Also, there needs to be a PredefinedGraphPane present (on the page, I assume) or the generator has problems. [#40210301]
create a UserCache singleton and proxy
added cucumber scenarios to check activities created from semantic json [40638719]
Updating runtime to line-of-best-fit
added factory girl
changed cucumber javascript driver from selenium to phantom.js
Merge branch 'master' into line-of-best-fit
BestFitSequences data_sets need > 5 data points