Fixes #542

Fix object existence check in common serialiser

See merge request !326

The check worked with SQLite but not with PostgreSQL.
In any case, since we have a user object here, there's no
reason to not use it directly.

Improve new version handling

Closes #547

See merge request !325

- beat.core to 1.10.3
- beat.backend.python to 1.7.6
- beat.cmdline to 1.7.0

- beat.core to 1.10.3
- beat.backend.python to 1.7.6
- beat.cmdline to 1.7.0

This mimicks the Linux behaviour and allows to properly run the
beat/beat.cmdline tests on macOS.

The original code would automatically generate a number based on
the previous version if any or set it to one. This has the drawback
of generating cryptic errors if for example someone sends an asset
with a version number higher than the one available on the platform
without providing historical data.

To avoid that, version is now a mandatory field. When doing local
development, it's already assigned. This allows for better handling
of posted data.

The refactored code makes only use of the database rather than mixing
prefix and database usage.

This makes things clearer and easier to understand.

Current code using these methods can be use as is while new
code can make use of the named fields.

Return compatibility information for algorithms and libraries

Closes #548

See merge request !324

It uses the new output of the "environemnts" method.