/plans/: Add an overlay to sign up.

[brockwhittaker]

This allows the form in /create_realm/ to be opened in an overlay rather than requiring the user to jump to a new page.

[zulipbot]

Heads up @brockwhittaker, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.

[zulipbot]

Heads up @brockwhittaker, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.

[timabbott]

Closing, since even if we plan to make sign up occur without navigating to a new page, we probably will want to do the implementation from scratch with the new brand colors and overall design. Thanks for the work on this @brockwhittaker!