Enable upload of different sound files with the same filename

[quimmrc]

Issue(s)
#1181

Description
Sound files uploaded with the same file name got overwritten in the description queue.
In order to solve that, my proposal is to change the filenames adding an index as done in most of OS files handling (for example, for download), e.g.:

a.mp3
a(1).mp3

Deployment steps:
upload() creates a submitted files list, using this list:

1. Create a new list to store filenames
2. For each file name, check if the name already exists in the list and if so, give it an index for as many times as the name has already appeared.
3. Update the filename with the index appropiately

Note: all filenames will be added to the filenames list for indexing purposes (for the duplicated ones, addition must be done before filename modification).

[ffont]

We should add a unit test for that, you can probably add an extra test method next to this one:

freesound/accounts/tests/test_upload.py

Line 44 in 8f29b21

def test_handle_uploaded_file_html(self):

[ffont]

Thanks @quimmrc! Once you have some time to add the unit test I'll test and merge this PR.

[ffont]

Hmmmm, this comment is a bit strange :S Could this be because in line 73 of test_upload.py you set "file" in singular?

[ffont]

Hi! @alastair just made me realise that this PR only addressed the case in which 2 files with the same name are uploaded in a single request, but does not address the case described in the issue, which is that a sound with that filename already exists in the user upload folder but it gets overwritten by a new one.

A way to fix that would be to add a check for duplicates which not only includes other files uploaded in the same request but also other files in user upload folder (here you can see how this folder path is defined). If there is a duplicate, then rename in a similar way as you are already doing. However, we might also simply enforce a "no duplicates" policy, let user know. If there is a filename which already exists in users' directory, then skip processing that file and don't call handle_file_upload, adding a new item to errors (see code here:

freesound/accounts/views.py

Lines 1302 to 1306 in 3ba8a17

	if handle_uploaded_file(request.user.id, file_):
	uploaded_file = file_
	successes += 1
	else:
	errors.append(file_)

)