Remove workspace class preference from user settings and only allow them on project settings

[gtsiolis]

Problem to solve

Following up from the relevant RFC (internal) about setting workspace class preference, let’s remove the possibility of setting workspace classes in the user settings and only allow them on projects.

Cc @svenefftinge @jldec @atduarte @Furisto

Proposal

This issue is focusing on the first step described in the relevant RFC:

MVC (Minimal Viable Change)

Step 1) Remove user-level setting for workspace classes, so project-level settings are the only way to change what is picked by default when starting a workspace.

Next Steps

Step 2) Introduce “Start with Options …” where we ask the user on a workspace class to start, to overrule whatever is in project settings.

Step 3) Have .gitpod.yml specify minimum requirements and in case the selected workspace class doesn’t fulfill them ask the user to pick a better one.