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):