Don't check usernames starting with an underscore (#1087)
Don't check usernames starting with an underscore (#1087)

The grammar allows such usernames. Even if it wasn't allowed
the server can return a different username. But some servers
might reserve the use of them.

Signed-off-by: Kurt Roeckx <bb87b47479d83cec3c76132206933257ded727b2@roeckx.be>
matrix-org/sytest,matrix-org/sytest