Change MC setup flow to have four steps

[eason9487]

Changes proposed in this Pull Request:

Partial of #863

• Add the fourth step page (layout only) for MC setup flow
• Change values of MC steps to string type and add the fourth step key/value
• Change MC setup flow to have four steps

Screenshots:

Detailed test instructions:

1. Go to the MC setup flow and it should have four steps
2. The button text of the third step should be "Continue", and click on it should move forward to the "Confirm store requirements" step

Changelog entry

[tomalec]

📜 In the latest designs "Confirm store requirements" headers is on a white background, whereas in this PR it's transparent/grey

[tomalec]

Reviewed the code, tested locally, run unit tests and linters (Travis is disabled for this branch). Left few comments about to-be-addressed later issues. LGTM

[eason9487]

📜 In the latest designs "Confirm store requirements" headers is on a white background, whereas in this PR it's transparent/grey

Opened a follow-up #924 for this.