Fix registration using GitHub

[VMelnalksnis]

Changes proposed in this pull request:

• Use the provider key as the user id if it is a GUID, otherwise create a new one

This fix is needed because GitHub provider key is an int.

[codecov]

Codecov Report

Merging #940 (3f41e35) into master (1d43bcd) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #940      +/-   ##
==========================================
- Coverage   72.97%   72.97%   -0.01%     
==========================================
  Files         427      427              
  Lines       11602    11604       +2     
==========================================
+ Hits         8467     8468       +1     
- Misses       3135     3136       +1     

Components	Coverage Δ
API	78.79% <66.66%> (-0.01%)	⬇️
UI	59.13% <ø> (ø)