merge IRC/Slack page

[cg505]

a lot of things (like channel lists) are duplicated between these two pages. To keep the page uncluttered, we can move the "how to connect to IRC" bits to another page. Discussed in #543.

[idfubar]

Please note Issue #702 also hints at the same issue (i.e. "cluttering", presumably by way of the 'More in this category' section); if we can find a better place for 'Services'->'XMPP' then perhaps that information could also be "collapsed in" to the IRC and/or Slack page...

(Also, a preliminary step could be something like PR #703