Official v3.0.0 release

It's been a long wait for this release. Almost all parts of NPBackup have been rewritten or have been updated.
A very long release candidate period has been observed in order to iron out all possible bugs and nipticks.
So here we are, proud to build some stairways to h... euhh easier but reliable backups.

This release is way too big to enumerate all the new functions. But here's a list of the major features:

• Multi-repository support
• Easier repo setups which can inherit from group settings
• New viewer application for viewing/restoring data from any restic repositories
• CLI and GUI are now two separate applications (still GUI can act as CLI)
• CLI can now fully operate in API mode via --json parameter
• New operation center which allows to run operations on multiple repositories and / or groups
• New housekeeping operation, which checks, implements retention policy and prunes backups
• Prometheus monitoring now supports all operations
• Upgrade server can now filter on client / group / platforms
• Memory and speed optimization
• Automatic tests

For the full changelog, please see CHANGELOG.md file.

Special thanks to @GuitarBilly for his help in finding bugs / asking questions / general help.