Create proposal of new community build design

[romanowski]

TODO:

• define mechanisms to define custom behaviour (targets and test tasks, plus custom scalac options)
• build graph using .ivy files
• build graph using sbt projects (with caching)
• measure coverage of updated deps on classpath
• compare both mechanisms and pick best for Open Community Build
• apply .sbt graph mechanism into Managed Community Build
• prepare reporting mechanism after each run of Open Community Build
• dist generated artefacts and prepare steps for easy reproduction of builds of single projects (include that in reports)
• add the possibility for a downstream community build to kick in
• allow companies to provide us with stats about their builds (with different degree of anonymization)