Only set the user password if the password field would have been shown

[zeripath]

POSTing to /admin/users/:id should only set/check the password if the the user IsLocal() or IsOauth2()

Fix #12952

Signed-off-by: Andrew Thornton art27@cantab.net

[codecov-commenter]

Codecov Report

Merging #12980 into master will increase coverage by 0.03%.
The diff coverage is 30.21%.

@@            Coverage Diff             @@
##           master   #12980      +/-   ##
==========================================
+ Coverage   42.54%   42.58%   +0.03%     
==========================================
  Files         670      671       +1     
  Lines       73522    73627     +105     
==========================================
+ Hits        31281    31353      +72     
- Misses      37175    37187      +12     
- Partials     5066     5087      +21     

Impacted Files	Coverage Δ
cmd/dump.go	0.90% <0.00%> (-0.13%)	⬇️
cmd/migrate_storage.go	0.00% <0.00%> (ø)
models/admin.go	69.09% <0.00%> (-2.61%)	⬇️
models/repo.go	55.15% <0.00%> (+0.04%)	⬆️
modules/storage/minio.go	0.00% <0.00%> (-48.44%)	⬇️
routers/admin/users.go	25.71% <0.00%> (ø)
routers/install.go	0.00% <0.00%> (ø)
routers/repo/lfs.go	0.00% <0.00%> (ø)
modules/storage/local.go	38.09% <5.88%> (-11.91%)	⬇️
modules/setting/storage.go	43.47% <43.47%> (ø)
... and 22 more

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