Create lassa Nextstrain build

[genehack]

This is a meta-issue to track the completion of the overall goal of creating and refining a Lassa build at nextstrain.org/lassa.

To launch build:

• https://github.com/nextstrain/lassa/issues/14
• https://github.com/nextstrain/lassa/issues/28
• https://github.com/nextstrain/lassa/issues/32

Next tier priority:

• nextstrain/lassa#20

Back log of active issues can be worked on live build:

• nextstrain/lassa#15
• nextstrain/lassa#16
• nextstrain/lassa#22
• nextstrain/lassa#26

[trvrb]

I just edited the above to make steps for live build more clear.