This commit makes the following structural changes to the web
client interface. No user-visible changes.
1. Splits login, legacy, readonly, and full management clients into
their own components, and pulls them out into their own view files.
2. Renders the same Login component for all scenarios when the client
is not logged in, regardless of legacy or debug mode. Styling comes
from the existing legacy login, which is removed from legacy.tsx
now that it is shared.
3. Adds a ui folder to hold non-Tailscale-specific components,
starting with ProfilePic, previously housed in app.tsx.
Updates tailscale/corp#14335
Signed-off-by: Sonia Appasamy <sonia@tailscale.com>
