Emulate CreateDoc if not implemented by backend

[flimzy]

This moves the CreateDoc implementation from each backend, into a single implementation in the frontend.

[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 64.04%. Comparing base (b8d1f63) to head (9a45baa).

Files	Patch %	Lines
db.go	81.81%	1 Missing and 1 partial ⚠️
internal/mock/db.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #896      +/-   ##
==========================================
+ Coverage   64.03%   64.04%   +0.01%     
==========================================
  Files         293      293              
  Lines       19519    19511       -8     
==========================================
- Hits        12499    12496       -3     
+ Misses       6441     6438       -3     
+ Partials      579      577       -2     

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