user invites: web: add "invite by sending link" functionality to welcome page

Created by: slimsag

Pretty simple change, when you are on the third stage of the welcome screen of Sourcegraph.com (i.e. you're waiting for your repositories to clone), there's now a way for you to copy a link to invite friends/coworkers to Sourcegraph:

When they visit that link, it's got your username in the URL ?invitedBy=slimsag and so it renders some decorations to show who invited you:

A few notes:

1. I did not fix the text color issue on the welcome page, that's not a new issue on this page. I will fix it in a follow-up PR.
2. I have not yet added event logging, I will do so in a follow-up PR.
3. Testing the welcome page (actually getting to it) is a huge pain, I plan to make that easier for us soon.
4. The cloud sign up page CSS is a mess, it's using a lot of negative margins and absolute positions.. I'd like to clean that up soon. @rrhyne can I also get rid of the signup-optimization feature flag which complicates this page? Do we want it on by default?

Up next, we'll make inviting people easier via sending emails and suggesting you invite collaborators from the repos you add.