execute "major changes proposal" plan

[nikomatsakis]

In a recent design meeting we drafted a "major change process". Let's make it happen.

Here are the steps:

• Create an issue template (see Create major_change issue template #232)
• Write-up a description of the process on forge so we can point people to it
• Author RFC major change proposal process for compiler team rfcs#2904
• Update template with final template from RFC
• Create Zulip stream t-compiler/major changes
• Create suitable labels for the various states of a MCP
• Create a bot that, when a MCP is opened, opens a topic in the Zulip stream?
• Extend rfcbot to support rfcbot second -- it can add a label MCP-Seconded
• And to respect rfcbot concern in the context of an MCP issue -- it can add (and remove) a label MCP-Concerns-Noted

cc @michaelwoerister @oli-obk