Use send only where necessary

Prefer public_send if methods are public,
and method calls if the method is a constant symbol.

`public_send` mentioned in bbatsov: https://github.com/bbatsov/ruby-style-guide#prefer-public-send

Under `app` and `lib`:

    find . -iname "*.rb" | xargs perl -lapi -e 's/\.send\b/.public_send/'

but some manual refactor was also done in addition.

Exceptions:

- `self.send` and just `send`. I moved `self.send` to just `send` for uniformity and to avoid hound barks. In those cases, the call can access private methods, so it does not matter.

- at `lib/redcarpet/render/gitlab_html` there is an `h.send` which causes problems is replaced... seems like we are using something we shouldn't. 
I can imagine why: a newb like me wrote that line =) (I've just learned about `public_send` while making this PR) Sometimes it makes me worry that GitLab lets me touch their code lol