Migrate to Legacy Builtin NDB Library

[phil-lopreiato]

Towards #3995

This will simplify a few things, mainly the fact we can just have native interop between old/new apps without having to do a bunch of hackery to get datastore seriliazation/memcache to agree on a data format.

The couple things still remaining:

• figure out if we can still use the remote api locally. Answer: no, because adding googledatastore creates dependency hell. We can file an upstream issue for this.
• update docs

[codecov]

Codecov Report

Merging #4018 (1827c40) into py3 (58e9f6c) will increase coverage by 0.58%.
The diff coverage is 99.90%.

@@            Coverage Diff             @@
##              py3    #4018      +/-   ##
==========================================
+ Coverage   93.05%   93.63%   +0.58%     
==========================================
  Files         422      418       -4     
  Lines       22715    21613    -1102     
  Branches       60       60              
==========================================
- Hits        21137    20237     -900     
+ Misses       1570     1368     -202     
  Partials        8        8              

Impacted Files	Coverage Δ
src/backend/common/logging.py	63.63% <0.00%> (ø)
...pi/handlers/helpers/tests/model_properties_test.py	100.00% <100.00%> (ø)
...backend/api/handlers/tests/add_match_video_test.py	100.00% <100.00%> (ø)
src/backend/api/handlers/tests/apiv3_auth_test.py	100.00% <100.00%> (ø)
src/backend/api/handlers/tests/cors_test.py	100.00% <100.00%> (ø)
src/backend/api/handlers/tests/event_test.py	100.00% <100.00%> (ø)
src/backend/api/handlers/tests/status_test.py	100.00% <100.00%> (ø)
src/backend/api/handlers/tests/team_test.py	100.00% <100.00%> (ø)
...backend/api/handlers/tests/trustedapi_auth_test.py	100.00% <100.00%> (ø)
...rc/backend/api/handlers/tests/update_teams_test.py	100.00% <100.00%> (ø)
... and 163 more

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