Channels are occasionally plagued by trolls who confuse users by
"hijacking" names of other users in the channel. This is accomplished
by replacing certain letters with visually similar letters (in fact,
indistinguishable in some sans-serif fonts), e.g. replacing lowercase
'l' with capital 'I'
This commit replaces capital 'I', lowercase 'l', digit '1', lowercase
'o', uppercase 'O', and digit '0' with '_' and changes the matching for
isUsernameTaken() to a LIKE query. Since '_' is a single character
wildcard, this causes the database to treat a username with one of these
simple replacements as already registered.
