Add batch generation to OTAGUI.

Batch generation of OTA packages is a important feature
requested by googler and partners: please refer to go/ota-dashboard-doc.
Given n incremental source builds and m target builds,
batch generation will generate n x m OTA packages in total.
If n=0, full OTA package will be generated.

The front end will be taking in the source/target lists and send the
request to backend one-by-one.

Test: mannual tested.

Change-Id: I769359ee69c7aa8c71704c4e119c374635554dfb
4 files changed