Summary:
- More const
- Use range-for
- Remove some redundant vars that weren't used anywhere
- Replace QString arg().arg() with QString::arg(string, string), (the
  latter is more safe to use)
- Where appropriate, move var declaration/definition to right where
  they'll be used

Test Plan:
make && a quick check with creating a channel on Freenode and setting
modes for it, seems to work well.

Reviewers: #konversation, psn, hein

Reviewed By: #konversation, psn

Differential Revision: https://phabricator.kde.org/D28357