Fix slug lookup regression (#587)

Fix slug lookup regression (#587)

Removing the Mongo InvalidID legacy code removed an explicit check for
"int" for the id lookup. This led the @get_user_media_entry decorator to
fail if we looked up a nonexisting non-numerical slug (it tried to query
the id with a string, which failed). Cast id to int and return 404 in
case it is non-numeric which fixes the regression. It does not fix the
underlying problem of slug_or_id lookups that were discussed.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>