Use Authentication API instead of scraping the SessionID

[palazzem]

Overview

Follow-up of #44

This PR introduces the Authentication API instead of using a form and parsing the SessionID from the web page. Introduced changes are:

• Uses <base_url>/api/login endpoint to grab the session_id
• A failed authentication raises HTTPError instead of PermissionDenied. PermissionDenied exception is used only when a session_id is not available
• Removes get_access_token()

[codecov-io]

Codecov Report

Merging #47 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #47   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          10     10           
  Lines         240    217   -23     
=====================================
- Hits          240    217   -23

Impacted Files	Coverage Δ
elmo/utils/parser.py	100% <ø> (ø)	:arrow_up:
elmo/api/client.py	100% <100%> (ø)	:arrow_up:
elmo/api/router.py	100% <100%> (ø)	:arrow_up:

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[palazzem]

This change breaks the Input/Areas endpoints because the client._session doesn't have cookies anymore. Before releasing this change we should include entirely the /api/strings endpoint.