Show executing tasks info

[fkorotkov]

In https://github.com/cirruslabs/cirrus-ci-web/commit/5f21f167dfb5270c6e0022b6d56b99377f918261 a new filed was added on a Build object. executingTasks implementation is very fast and basically no-op for builds that have finished.

Problem right now is that when you filter out on running build on the home page, it's still hard to reason about resources that are used by the tasks. What we can do instead is to accumulate all the instanceResources of all the executingTasks probably grouped by instancePlatform and instanceArchitecture and show it next to the builds.