add content gaps files

content_gaps.py

• add get_wikidata_qitems to extract all wikidata qitems
• add get_wikidata_properties to extract all property-value pairs for all wikidata qitems
• change aggregate_item_property to multiple functions. one function handles one property
• change get_wikipedia_revision_text: remove redirect pages by joining mediawiki_page table

content_gaps_metrics.py

• show the flow of getting one big dataframe (not including article features yet)
• examples for computing metrics
• examples for plotting graph

article_features.py

• wrap multiple wikitext functions from Isaac's example code to one udf