Extend data model by the concept of durations

[muety]

Pretty much analogously to WakaTime's durations. A duration will conceptually be "in between" of raw heartbeats and aggregated summaries and especially allow for filtering (see #108). It's multiple heartbeats (probably within the time span of one day, as this is the smallest time unit we want to support) aggregated by all entity types, i.e. approximately the result of GROUP BY on (project, language, editor, os, machine) (+ (entity, branch) after implementing #242, #80). We might actually implement their computation in SQL. Durations will then be aggregated to summaries. Summaries shall then only be stored for the purpose of caching, but not actually be relied upon in business logic.

• WakaTime durations example: durations.json
  • (https://wakatime.com/api/v1/users/current/durations?date=2021-10-13)
• WakaTime summaries example: summaries.json
  • (https://wakatime.com/api/v1/users/current/summaries?start=2021-10-07&end=2021-10-13&project=my-new-project-1&timezone=Europe%2FBerlin)
  • As explained here, entities and branches fields are only included if a project filter is applied. 💡 Non-project-specific summaries are actually summaries of summaries, i.e. multi-project summaries of project-specific summaries (the latter still include entity- and branch information).