Use SlugField in all model identifiers

[ghoshbishakh]

Identifiers are used in urls. So using a slugfield instead of custom regex will be better.

https://docs.djangoproject.com/en/1.10/ref/models/fields/#slugfield

So tasks are:

• Change Event identifier https://github.com/NIT-dgp/festcms/blob/master/festflow/models.py#L20

• Change Keynote identifier https://github.com/NIT-dgp/festcms/blob/master/festflow/models.py#L57

• Change About identifier https://github.com/NIT-dgp/festcms/blob/master/festflow/models.py#L147

• Change NewsLetter identifier https://github.com/NIT-dgp/festcms/blob/master/festflow/models.py#L160

• Change Subscription identifier https://github.com/NIT-dgp/festcms/blob/master/festflow/models.py#L170

• Change FAQ identifier https://github.com/NIT-dgp/festcms/blob/master/festflow/models.py#L187

• Remove the regex: https://github.com/NIT-dgp/festcms/blob/master/festflow/models.py#L11