url with sha1 in uppercase (or mixed upper-lower case) was gives 404.
Convert snapshot_id to snapshot_id.lower(), so that SHA1 remains lowercase consistently. I will also work with SHA1 with mixed upper and lower cases.
Related T1505
> Currently passing a sha1 as url argument to the webapp endpoints works only if it is in lowercase form.
> If a sha1 is passed in uppercase form, we end up in a 404 error page (see [1] for instance).
>
> For commodity of use, we should support passing sha1s in uppercase form but also with
> mixed uppercase and lowercase parts.
>
> Redirecting to the lowercase sha1 endpoints when such cases are encountered seems the
> right way to implement this.
>
> [1] https://archive.softwareheritage.org/browse/snapshot/1A8893E6A86F444E8BE8E7BDA6CB34FB1735A00E/