Added fix to disable unauthorized access by volunteers

[Yureien]

Fixes issue #326

In addition to the links mentioned in the issue, I've also fixed these urls -

1. /volunteer/edit/ - Previously could be accessed by anyone
2. /volunteer/add_hours/ - Previously could be accessed by anyone
3. /volunteer/edit_hours/ - Previously could be accessed by anyone
4. /shift/cancel/ - Previously showed an Http 403 page, fixed to show same error page as other ones.

Here is the screenshot -

PS: In this commit, administrators cannot see the volunteer restricted pages too except shift sign-up page. If this needs to be changed, please tell me.

[Yureien]

There are some more test errors, I'll fix them by tomorrow

[coveralls]

Coverage increased (+0.2%) to 91.531% when pulling 024fcdd on InfernoCoder:VOLUNTEER_URL_FIX into 46171dd on systers:develop.

[smarshy]

Thanks for the work and especially for spotting the extra urls. This seems to be working fine.

[smarshy]

@InfernoCoder There are conflicts in this PR. Please resolve them. Let me know if you need help

[coveralls]

Coverage decreased (-0.03%) to 91.72% when pulling a1daa4e on InfernoCoder:VOLUNTEER_URL_FIX into 880a3c8 on systers:develop.

[Yureien]

@smarshy Fixed the conflicts

[Yureien]

This PR is closed and moved to #449 .