Extract keyword metadata from RSTs to Splice metadata export (#163)

* including export controller for generating export metadata for opendsa embeddable exercises for SPLICE

* including routes to view exported metadata, see endpoint /embed/export

* including 3 functions to extract data from avmetdata and inlineav directives from opendsa/rst

* updated export controller access data like avmetadata from inst_book extract methods from avmetadata in RST files

* including 4 methods to access RST raw files, extract data from avembed, avmetadata and inlineav

* updating export controller to collect keywords data from extract

* Update export_controller.rb

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Co-authored-by: Alex Hicks <awh4kc@vt.edu>