A task force member is required to manually call
https://apis-explorer.appspot.com/apis-explorer/?base=https://cr-buildbucket.appspot.com/_ah/api#p/swarmbucket/v1/swarmbucket.set_next_build_number
to set the next build number to latestBuildbotBuildNumber+10.

We can automate this. Feature request:
- replace the "LUCI is Prod?" checkbox with a button with alternating text "make LUCI prod" and "make Buidlbot Prod" (or some other text with the same meaning)
- when "make LUCI prod" is clicked, the migration app server should figure out the latest buildbot build number and call the aforementioned API for the user
- when "make Buildbot prod" is clicked, the client-side popup should ask whether the user has already bumped the next build number on buildbot and restarted the master.