Support underscores in MMF usernames

[Floppy]

Fixes #4632

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00% (target: -1.00%)	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (1675e40)	5149	4361	84.70%
Head commit (fc114f4)	5149 (+0)	4361 (+0)	84.70% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4640)	1	1	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

See your quality gate settings    Change summary preferences