Restricting the receipt API to authenticated users (bug 917601)

Otherwise a 500 will be raised given the usage of `request.amo_user`
Don't apply filters to Collections API when pk/slug/etc lookup is used (bug 915801)
Merge pull request #1163 from diox/collections-dont-filter-lookups

Don't apply filters to Collections API when pk/slug/etc lookup is used (bug 915801)
Commit from Mozilla Verbatim by user petercpg@mail.moztw.org.: 5047 of 5474 strings translated (0 fuzzy).
Merge branch 'master' of ssh://github.com/mozilla/zamboni
Commit from Mozilla Verbatim by user mdinic@mozilla.com.: 406 of 5474 strings translated (2 fuzzy).
remove paypal check
Expose fallback used  in Collections API (bug 914318)
Merge pull request #1165 from diox/collections-expose-fallback

Expose fallback used  in Collections API (bug 914318)
Commit from Mozilla Verbatim by user petercpg@mail.moztw.org.: 5086 of 5474 strings translated (0 fuzzy).
Merge branch 'master' of ssh://github.com/mozilla/zamboni
Commit from Mozilla Verbatim by user petercpg@mail.moztw.org.: 5087 of 5474 strings translated (0 fuzzy).
Commit from Mozilla Verbatim by user petercpg@mail.moztw.org.: 5087 of 5474 strings translated (0 fuzzy).
Removed unused tasks.

See commit af6e6a7896cd9e5aa51d017f2abace084938669e.
check for image presence without database hit (bug 918522)
pyflakes cleanup
don't run files languagepack tests twice
Merge pull request #1159 from davidbgk/917601-receipt-api-500

Restricting the receipt API to authenticated users (bug 917601)
Allow collection images to be deleted (bug 917081)
Merge pull request #1170 from chuckharmston/917081-clear_image

Allow collection images to be deleted (bug 917081)