uncategorized bookmarked sounds download

[quimmrc]

Issue(s)
#1290 - uncategorized bookmarked sounds

Description
Make available the download of uncategorized bookmarked sounds for a user.
New view function to handle these type of sounds, which since they do not correspond to a BookmarkCategory object they need slight changes in their handling.

Deployment steps

1. Download uncategorized bookmarks id set to 0 (URL handling).
2. download_bookmark_category view computes necessary information according to category_id
3. uncat_bookmark_data returns all necessary information to download uncategorized bookmarks. It only uses the "request" parameter because it needs to be called both from "download_bookmark_category" and "bookmark_category_licenses" views, and I did not know how to handle the latter with suitable parameters for the purpose.
4. bookmark_category_licenses view returns url according to category_id
5. .txt template contemplates receiving "not_an_object" for its rendering, since uncategorized bookmarks do not group in any object itself (I think).

[quimmrc]

Further work to do:

add unit tests for licenses views (one for packs and one for bookmark categories): implemented in the last commit

[ffont]

Thanks for the PR! After our discussion at the office we decided that we would not merge this PR as it adds a bit of weirdness and we'll replace this with collections "soon" anyway.