Use a "cleaner" design for the dashboard, without cards

[acelaya]

So far, we have been using a design for the dashboard that matches what we have been using in other pieces of UI, like the file picker, or the email settings app. This allowed us to use existing reusable components without applying big customizations.

This PR moves the dashboard to a look-and-feel closer to the one from the designs, removing the grey background and the use of Cards.

[!NOTE]
This PR is easier to review ignoring whitespaces

Before:

After:

[acelaya]

~the table's grey header and their wrapper's lighter grey do not have enough contrast. We may need to change the header style to also be closer to the designs.~

EDIT: Designs have been updated to address this.