Copy text improvements

- Translate non-translated strings in the accept transaction form
- Mention minimum fee and PayPal's fees in payment preferences info text
- Move text that has been proofread to getting started page
- Link to marketplace home page from getting started page
- Mailto link to support to getting started guide
- Mailto link to sales to getting started guide (custom domain)
- "No received messages" -> "No messages" in inbox
- Improve welcome email copy about "are you new to online marketplaces"
- Mailto link to support to welcome email
- Mailto link to support to left navi of admin panel