Rename "Starter" plan to "Basic"

The "Starter" name was chosen for the plan as we envisioned that customers would move up plans as they grow more advanced and large. However now this seems to be more of a rare case, usually customers of a specific type will choose a plan and stick with it (Farshad Tavakoli has done more extensive analysis on this).

The name "Basic" is more suited in this new model, so we would like to rename it. Note that the plan name is shown in several systems to the customer (at least website, UI, billing) and we need to coordinate the change across all (esp. customer facing) systems.

User value (why)

• Easier plan selection as it better describes who is it for

Acceptance criteria

• Plan is renamed from Starter to Basic in
  • UI
  • Website
  • Documentation 
  • Billing and Invoicing system
  • Any relevant internal systems
• An announcement about the change is sent to all hosted Mender customers (clearly pointing out that the name of the plan is the only change to avoid fear of new round of plan changes)