Advertise Event API support

[r-marques]

This pr adds information to the api info endpoint about the streams of the event api.

{
  "_links": {
    "docs": "https://docs.bigchaindb.com/projects/server/en/v0.10.0.dev/drivers-clients/http-client-server-api.html", 
    "self": "http://127.0.0.1:9984/api/v1/", 
    "statuses": "http://127.0.0.1:9984/api/v1/statuses/", 
    "streams_v1": "ws://127.0.0.1:9984/api/v1/streams/", 
    "transactions": "http://127.0.0.1:9984/api/v1/transactions/"
  }
}

[codecov-io]

Codecov Report

Merging #1356 into events-api-first-cut will increase coverage by <.01%.
The diff coverage is 100%.

@@                   Coverage Diff                   @@
##           events-api-first-cut   #1356      +/-   ##
=======================================================
+ Coverage                 98.19%   98.2%   +<.01%     
=======================================================
  Files                        54      54              
  Lines                      2444    2447       +3     
=======================================================
+ Hits                       2400    2403       +3     
  Misses                       44      44

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