Allow creating and updating (user|org) names with dots

Created by: tsenart

In #4690, we made it possible to create usernames from SSO signups that include dots. But that would have failed, as I found out working on this, due to a database constraint that needed to be updated.

Additionally, it was still not possible to change or manually create an account with a username that includes a dot in the UI. This PR fixes that too.

For consistency, we allow dots in org names as well.

Fixes #4909, #4920

Created by: codecov[bot]

Codecov Report

Merging #4925 into master will decrease coverage by <.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4925      +/-   ##
==========================================
- Coverage   48.08%   48.07%   -0.01%     
==========================================
  Files         732      732              
  Lines       44645    44645              
  Branches     1763     1763              
==========================================
- Hits        21467    21465       -2     
- Misses      21211    21213       +2     
  Partials     1967     1967

Impacted Files	Coverage Δ
cmd/frontend/graphqlbackend/repository.go	22.05% <0%> (-0.99%)
cmd/frontend/graphqlbackend/graphqlbackend.go	48.95% <0%> (ø)

Created by: keegancsmith

@tsenart I cherry-picked this onto the 3.6 branch.