user invites on homepage: ship to on-prem customers

Created by: slimsag

We shipped user invites on the homepage for all Sourcegraph.com Cloud users, with the intent of reviewing it's impact and collecting feedback for 2 weeks before shipping to on-prem.

Before we ship to on-prem, we should:

• Verify the impact this change has had: For the entire month of March, just 52 signups came through this. https://analytics.amplitude.com/sourcegraph/chart/d4oba5a?source=workspace
• exclude existing users based on name, not just email (~1 week est.)
• Decide if we should continue investing in this.

My opinion: wait to see what signup % looks like in April, if ~10% then we should continue refining this. Otherwise, it is my belief that the current ~3.49% of signups it brings does not justify the real estate it takes up on the homepage and concerns it may create about brand.