Rename keepassx2-http to keepassx-reboot

The `keepassx2-http` fork has been moved to a new organization and
renamed to `keepassx-reboot`. For more details on the change, see the
discussions in GitHub issues [1][2].

Included changes:
- Rename the `keepassx2-http` package to `keepassx-reboot`
- Fetch source from correct (moved) GitHub repository
- Update the version to the latest release
- Change the `homepage`, as these projects are likely to diverge over
  time
- Add `keepassx2-http` to `aliases.nix

[1] https://github.com/keepassx/keepassx/pull/111#issuecomment-250639109
[2] https://github.com/keepassxreboot/keepassx/issues/40