feature-request: add “Other Downloads” section

[ericswpark]

Feature request description

Add an "other downloads" section for devices

Additional context

Things like recovery.img and fastboot zips

[ericswpark]

Google has recently enforced signing all build artifacts, which means stuff like the recovery and kernel images must be signed as well. This results in a recovery-package.zip file after the build process is complete, which the user must flash.

This package changes with each compilation, and significant deviations from the previous zip can cause the device to not work if the recovery package does not match the build zip.

To facilitate this new change, we need the following implemented:

• Other downloads associated with each device (e.g. bootstrap files)
• Other downloads associated with each build
• shippy uploads for a recovery-package.zip file with each build file.
  • shippy should not offer this if there are multiple build files
  • shippy should ask before associating a recovery-package.zip file with a build file
  • Relevant APIs added to backend