Otherwise, some attributes set from ECODES will be kept in the dict.

That is a valid way of resetting and must be properly regognized.

Do that so we get the correct order of operations: as the comment says,
append_themed_infix(infix) must use the previous (a.k.a. current)
attr dict.

Extract existing logic to get ANSI CSIs into a dedicated module. While
at it, also implement a function to filter out those escape sequences.
That function will be used in a future commit to clean up the message
text when generating the quoted text for the a reply.

It is undesirable to have those sequence in the quoted text of a reply
message. Remove them.