Seek back to the beginning when uploading a file (bug 1045764)

Some of our code might read the beginning of the file returned by
the form before handing it over to FileUpload.from_post().
Commit from Mozilla Verbatim by user kohei.yoshino@gmail.com.: 1966 of 1966 strings translated (0 fuzzy).
Merge branch 'master' of ssh://github.com/mozilla/zamboni
Merge branch 'master' of ssh://github.com/mozilla/zamboni
Commit from Mozilla Verbatim by user petercpg@mail.moztw.org.: 1966 of 1966 strings translated (0 fuzzy).
Commit from Mozilla Verbatim by user cesperanc@.: 1966 of 1966 strings translated (0 fuzzy).
Commit from Mozilla Verbatim by user civas.: 1675 of 1966 strings translated (0 fuzzy).
Commit from Mozilla Verbatim by user markh@babelzilla.org.: 1966 of 1966 strings translated (0 fuzzy).
Commit from Mozilla Verbatim by user civas.: 1704 of 1966 strings translated (0 fuzzy).
seek might not be present, chunks can be a list in some cases
Merge pull request #2403 from diox/fix-api-packaged-apps

Seek back to the beginning when uploading a file (bug 1045764)
Use new feed palette, move validation from database to API (bug 1050496).
Merge pull request #2413 from chuckharmston/1050496

Use new feed palette, move validation from database to API (bug 1050496).
Fix tests broken in 60953ba
Merge pull request #2415 from chuckharmston/1050496

Fix tests broken in 60953ba