- instead of storing the signature of an author in the `signature` attribute of commits, a connection is used
- the frontend still uses the `signature` attribute, but it is inferred using the connection
- #145

- instead of using the `mentions` attribute in issues
- #145

- commits were not sorted with new localDB function, which broke the Changes/Version mode of the Code Hotspots visualization.
- #145

- instead of the `sha`/`ref` attributes in the builds collection, the commits-builds connection is used.
- #145

- #145

- Because the `builds` connection does not have a `sha` attribute anymore, the Histogram endpoint was broken both in the serverDB and the localDB.
- #145

- instead of using the `history` and `parents` attributes, the commits-commits connection is used.
- the graphql service infers the parents directly, as does the localDB when accessing commits. This means the `parents` attribute can still be queried, even if it's not in the DB anymore.
- query times are much faster, DB export files significantly smaller
- #145

- fixed bug where filepicker crashes when new files are added in the project directory but ownership has not yet been indexed

- when querying commits using the seerverDB, commits that are earlier than the specified earliest date should only be discarded after the history has been calculated for all commits. Otherwise, history calculation would not work properly.
- #145

- fixed bug where sprint visualization crashes when no sprints are defined
- also fixed minor styling/behaviour bugs of new TabCombo component

- fixed bug that led to test fails