Move getTabContext and getUserlistContext into menus.js for cZ

The only caller for getTabContext and getUserlistContext is now in menus.js, so might as well move these helpers into that file. Do some tidy up of their code whilst here.

[Approval Request Comment]
Regression caused by (bug #): n/a
User impact if declined: none
Testing completed (on m-c, etc.): 2.53.22pre
Risk to taking this patch (and alternatives if risky): low
String changes made by this patch: none

Comment on attachment 9479910 [details] [diff] [review]
1961385-irc-context-move-25322.patch

LGTM r/a+

Pushed by frgrahl@gmx.net:
https://hg.mozilla.org/comm-central/rev/59d287b3617d
Move getTabContext and getUserlistContext into menus.js for cZ. r=frg DONTBUILD

https://gitlab.com/seamonkey-project/seamonkey-2.53-comm/-/commit/b7ea5118ca10f41bb8fc1610b42360146c824345
Move getTabContext and getUserlistContext into menus.js for cZ. r=frg a=frg