Discord migration

[adambirtles]

As decided by the Committee, we're officially moving to Discord! This PR updates the website to reflect that change.

The most significant change is to the chat platforms page (templates/content/irc.html.jinja2). Additionally, various references and links to Slack on a few pages (and in some bits of documentation) have been replaced with Discord.

Additionally, this updates the Code of Conduct, which will soon be changed by HackSoc/constitution#23.

Changes still needed

This PR is not quite ready to be merged. Some large changes to the Code of Conduct's Guidelines and Conventions section are still required:

• In general, the entire section needs to be changed to treat Discord as the primary platform.
• The Bridging subsection needs to be rewritten. It should:
  • Explain which channels on Discord are bridged to which channels on Slack and IRC.
  • Discord channels that are bridged say so in their descriptions.
  • The bridged channel’s name is only stated when it differs from the Discord channel’s name
  • Describe the behaviours of the new bridge.
• The Content Warnings subsection should specify how we will use them on Discord (spoilers).
• The Profiles subsection should specify that people are welcome to state their pronouns
  • The can add them to either their profiles and/or their nicknames.
  • Server-specific profiles are a premium feature so they should use their server nickname if they don’t wish to be out to the entire world.
• It should probably mention somewhere that Discord's channels are opt-out by default.
  • This can be done with muting.
  • The big exception is #politics, which is opt-in using a role.

[adambirtles]

Only waiting on passing the amendment to the constitutional appendix now. Then this can be merged.