Simplify rebalance ui

[alistair-marshall]

Previously the rebalancing was sorted through two separate questions. This combines the options into one pull down menu:

• No rebalancing
• Constant allocation
• Glidepath

Current status:

• [x] Update UI to include only one pull down box.
• [x] Update the code to use the new UI
• [x] Clean up the code using the simpler options available
• [x] Update the help documents to reflect the new UI