Added medical image viewer option

Added option to enable the medical viewer in the competition form. This option enables the following workflow:
* An extra column is added to the results leaderboard.
* A url is generated, which passes the location of the input_data and the path of the submitted results to the viewer (running as a separate webservice on azure).
* A sas key is generated to access the private blob storage account which is valid for 60 minutes.

Major changes in existing files/functions:
* Renamed codalab.py file to codalab_tags.py to be able to import "from codalab.azure_storage import make_blob_sas_url" (couldn't manage it otherwise).
* Modified function make_blob_sas_url() to pass "r" or "w" permission as an argument.
* Added resultLocation to phase.scores() to save the file location of submitted results.
* Adapted _results_page.html to reflect these changes.

use `enumerate` instead of a separate counter

Example from python docs:
```python
def enumerate(sequence, start=0):
    n = start
    for elem in sequence:
        yield n, elem
        n += 1
```