Forbid unparseable facts

Created by: ederag

Make the save button active only if the fact entered can be parsed (and thus edited) from the cmdline. This hopefully answers https://github.com/projecthamster/hamster/pull/461#issuecomment-549302989 favorably, while waiting for #465 (closed).

The first two commit are from PR #475, only a544d5e is added by this PR.