Remove version.json from repository

[hackathi]

version.json essentially is a build artifact, that has no place to be checked in with master. In essence, it is a moving target, and each and every commit must touch that file.

For releases, this is not a big deal: just generate it using make release. However, the code currently has a hard dependency on the contents of this file to generate cache invalidation additions.

In a development scenario, this logic is fundamentally flawed anyways; I want reloads on every request that the actual file has changed.

So my proposed solution would be:

1. nuke version.json from the repository, but generate an manifest file on every build.
2. The version of the manifest is determined by git describe --tags
3. Stop using the version as a cache invalidation method in the url. Instead use sha1(sha1(grocy.js) + sha1(grocy.css)) as a content hash. This has the slight downside of reloading the cache if either grocy.js or grocy.css change, but is stable in scenarios where not-grocy is installed from git and upgraded between releases.
4. Put all these things into a php file manifest.php and include that. There is absolutely no reason to use json here, this file isn't consumed by anyone.

[hackathi]

Scope expansion: remove the hard dependency on git in PATH if running in prerelease mode, which actually figures out the version string stuff already.

See also Grocy\Services\ApplicationService::GetInstalledVersion().