[dashboard] Update What's New modal

Problem

Back in https://github.com/gitpod-io/gitpod/issues/3297 and https://github.com/gitpod-io/gitpod/pull/3618, we introduced a What's New modal to inform users about new changes to the dashboard and offer the option to switch their editor to VS Code.

Some more context:

1. The initial plan was to keep this modal for one release and remove it on the next one.
2. In this release we want to inform users about the new dashboard theme that is going to be available.
3. Users should be able to get back to this modal using a What's New button in the profile dropdown.
4. The updates that go in this feed can be quite useful for the Changelog feature we're adding for our website.

Proposal

Let's:

1. Keep the What's New modal and figure out a scalable way to append new updates as well as hide past entries.
2. Use this modal to add an update about the dark theme availability.
3. Hide the past two entries (New Dashboard and Editor Switch) and attach the March 2021 milestone timestamp.
4. Introduce a What's New button on the profile dropdown so that users can trigger the What's New modal.
5. To avoid introducing too much noise, add a small dot indicator on the profile to indicate there's a new update.

Designs

TBD (To be discussed)

Start with this issue and the linked issue #3297 and pull request #3618 to understand the existing What's New modal. The designs are still marked TBD, and the proposal spans update storage, modal entries, profile-dropdown access, and a new-update indicator; done means these behaviors are designed and implemented, with past entries hidden and the March 2021 timestamp attached.