Implements logic for #304

[shankisg]

Implements logic for #304

This implementation allows tutor to request number of volunteers required, ranges between 1-5. On click of accept button on workshop details page, the tutor will see a popup for requesting number of volunteers required.

Depending on volunteer requirement, user with usertype volunteer can opt-in as volunteer. Also volunteer will have the ability to opt-out as volunteer.

**For using this feature, we need to add usertype as volunteer in the database.

[coveralls]

Coverage decreased (-55.6%) to 35.176% when pulling 98392ee on shankisg:WYE-304 into e6b0c69 on pythonindia:development.

[vnbang2003]

@shankisg Something looks wrong . All test cases are failing . Can you check it ?
Probably easy would be create new branch from development and apply you changes to it.

[shankisg]

@vnbang2003 I will check why the testcases are failing. Quick feedback, it seems the test environment is upgraded to Python3.X, and in number of places it is failing for xrange which is not available in Py3.X

File "/home/travis/build/pythonindia/wye/wye/workshops/forms.py", line 193, in WorkshopVolunteer
    CHOICE_LIST = ((idx, idx) for idx in xrange(0, 6))
NameError: name 'xrange' is not defined

[vnbang2003]

We have been running test on python3.5 for long
Latest check ins:

https://travis-ci.org/pythonindia/wye/builds/187705901
https://travis-ci.org/pythonindia/wye/builds/187705748

[vnbang2003]

@shankisg I guess you local environment is Python2 and hence you didn't see it failing .
Can you please upgrade to python3.4 + version .
We will be soon migrating wye to version python 3.6 and django 1.10(Branch:pyup-initial-update)

[coveralls]

Coverage decreased (-0.5%) to 90.215% when pulling 839fa75 on shankisg:WYE-304 into e6b0c69 on pythonindia:development.

[vnbang2003]

@shankisg I have created new branch : volunteers-feature let use this .