Add loc_usa and a couple of other channels

Why is this change being made?

I notice now and again "USA as default" at GitLab - a small example is posting messages meant for only USA team members in #whats-happening-at-gitlab or other Slack channels meant for everyone.

In order to reduce noise https://about.gitlab.com/handbook/communication/#be-respectful-of-others-time and increase inclusivity of non-USA GitLab team members, I propose creating #loc_usa channel, to which messages about USA-specific benefits, tax questions and other issues can be posted. If there are events affecting USA, they can be posted in that channel. As we scale, the attention economy of an increasing number of team members becomes more critical, it makes sense for increased efficiency for those announcements to go in a specific channel for GitLab USA.

A similar example of usage is #loc_canada, which mainly includes general chat relevant to Canadians such as forms and GitLab Canada Corp specific messages and information. Not much chitchat happens in that channel due to the size and variation of Canada communities, so most chat and relationship building occurs in local channels such as #loc_bc (British Columbia).

Process changes:

• Add #loc_usa Slack channel, add description
• Edit link in this MR to point to that new channel
• Ensure any new USA based team members are added to this channel
• Add all team members based in USA
• Announce in #whats-happening-at-gitlab slack channel so anyone else can join the channel if they wish

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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