Implement server-side download hash computing

[Calinou]

The server should compute the SHA-256 checksum of all version downloads (both if the link is inferred from the version tag or custom). It should then be persisted to the database.

This hash could also be displayed with small text on asset detail pages.

[Calinou]

Note that a workaround was implemented in 8ac10f9, so the API should still be functional for now.

That said, consider this comment:

/**
 * [...]
 * TODO: Consider computing the download hash on a per-version basis and store it.
 *       However, this may not be a good idea as GitHub ZIP hashes aren't guaranteed
 *       to be the same over a long period of time.
 */

[bojidar-bg]

As hashes were removed in the official asset library (currently replaced by using exact commit hashes when specifying a download link), I suppose this can be closed?

[Calinou]

@bojidar-bg Indeed, thanks for the reminder 🙂