common: Sort user prefix channelmodes on add/set

Sort prefix channelmodes whenever adding/removing/setting per-channel
IrcUser modes.  This keeps the highest mode at the front, necessary
for backlog storage to allow only showing the highest mode.

It's not possible to get away without sorting as modes can be added
in any order.

modesToPrefixes/prefixesToModes operate in order, so sorting prefixes
results in sorted modes for free.  IrcUser modes don't need sorted as
they're not hierarchical and not stored anywhere.