Generate source keys when journalist flags them

Closes https://github.com/freedomofpress/securedrop/issues/140 with the approach described in the original ticket. 

Screenshots:
1. Journalist sees button to Flag a source for reply in col/
![screenshot from 2013-11-11 10 13 09](https://f.cloud.github.com/assets/549654/1515318/8e85c2a8-4b00-11e3-9dec-49b06615c4bf.png)

2. Journalist view after clicking Flag
![screenshot from 2013-11-11 10 13 16](https://f.cloud.github.com/assets/549654/1515322/a41aecba-4b00-11e3-88d5-77bad5f871b4.png)

3. Journalist view of collection page for a Flagged source
![screenshot from 2013-11-11 10 13 27](https://f.cloud.github.com/assets/549654/1515326/b9730d9a-4b00-11e3-9ae4-00e818127812.png)

4. Flagged source view after logging in. At this point, a key generation is in progress for the Source.
![screenshot from 2013-11-11 10 14 04](https://f.cloud.github.com/assets/549654/1515339/df3fd9d6-4b00-11e3-96b6-c7a42f9f3306.png)

5. Journalist view of collection page for a Flagged source after keygen is finished.
![screenshot from 2013-11-11 10 14 20](https://f.cloud.github.com/assets/549654/1515345/feb6e782-4b00-11e3-9a12-4e7f1ad87ce0.png)