Add text notification of answers

[cndreisbach]

For users who have burning questions, they may want to receive a text notification when their question is answered. You will use SendHub or another texting service to send text messages. (I chose SendHub because they have a free trial that doesn't restrict what phone numbers you can text.)

Requirements

• Users can add their phone number to their account.
• Users can decide if they want to get a text message when their question is answered. This should be turned on and off from the questions/show page by clicking a button or a phone icon or something like that.
• If they have chosen to get a text message, then when an answer is added, they will get a text message with this message: "New answer for your question '[original question title text]': [truncated answer text]"
• Texting should be tested, but no texts should be sent during testing. Read up on mocks and stubs and see the MiniTest documentation on how to use them.

Bonuses

• Verify phone numbers before allowing texts to be sent to them.
• If the user responds to a text with the message "off", then text notifications for that question are turned off.