Migrate mass-assignment security to strong_parameters

Ahead of the upgrade to Rails 4, this change removes the use of
attr_accessible and attr_protected as mass assignment protection
mechanisms in favour of whitelisting input attributes at the
controller level.

See http://blog.markusproject.org/?p=5588 for more details.

All tests are passing, and limited playing around with a local server seemed to work correctly. However, I am not confident that I have get the assignments controller completely working and there may be cases that the tests do not cover. So, I am opening this PR for review only at this time.
Given the following code
```ruby
{
  a: {
       b: 1
     }
}
```
Rubocop also complains about this style:
```
1.rb:3:11: C: Use 2 spaces for indentation in a hash, relative to the start of the
line where the left curly brace is.
          test: 1
          ^^^^^^^
```
I think the acceptable styles are
```ruby
{
  user: {
    test: 1
  }
}
```
Or
```ruby
{ a: 1,
  b: { c: 1,
       d: 2 } }
```