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:

• Push to production rather than staging after completing automated build #14
• Push to live #28
• Get live pathogen build onto nextstrain.org #32

Next tier priority:

• Request to add a GPC Phylogenetic Tree  #20

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

• Add frequencies plot #15
• Deal with duplicate strain names #16
• Consistently pick a reference strain for all builds #22
• Add logs and benchmarks for most rules #26

[trvrb]

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