support searching TV News via IA/GDelt API

[rahulbot]

We want to support searching the Internet Archive's television US news archive for national TV minutes that include text matching a user query. The first step will be to integrate the GDelt/IA television API (full docs).

Notes:

• This should be implemented as a new subclass of ContentProvider
  • Attention: It looks like we can get attention over time and total attention in minutes
  • Language: It looks like we can get words counts
  • Sample: Perhaps this should be links to clips in IA? Or can we get an extract of the matching text too?
• This should include a unit test in glimpse/server/platforms/test
• Include appropriate constants and logic in glimpse/server/platforms/ to handle it like other platforms for queries coming in