Fix for Issue 1759

**[Issue #1493](https://github.com/MarkUsProject/Markus/issues/1759) Spreadsheets: "Select: All" doesn't select all**

**Summary**
* When you click "Select: All", no checkboxes get checked. Then when you try to "Release Marks", it doesn't work.
* If you click "Select: All" and then "Select all XX items", no checkboxes get checked. But "Release Marks" button works.

**Fix**
* In the checkbox switch function, the DOM get function was looking for the wrong class. There was an extra period in front of the class name.
* Also in the checkbox switch function, it was setting the 'value' attribute between true/false. However, it should add the 'checked' attribute instead.

**Tests**
* Ran test suite. There was one error that is associated with user permission that was present on a clean repo clone.
```
[396/691] MarkusLoggerTest#test_: A MarkusLogger instance should raise exce        

1) Failure:
MarkusLoggerTest#test_: A MarkusLogger instance should raise exception if logfile is in a directory with no writing permissions.  [/Markus/test/unit/markus_logger_test.rb:204]:
MarkusLoggerConfigurationError expected but nothing was raised.
```
* Checked that all the checkboxes are selected when "Select: All" is clicked.
* Checked that after "Select: All" is clicked, marks can be released to all of them. Pass Condition: Last column indicates that marks are released.
* Checked that "Select: None" clears all the checkboxes.