set mixeval questions to default to required.

This change was made for the convenience of the instructor. This is
so that they don't have to check off each question as required.

I have also added help-text for likert questions reminding the user
that unrequired likert questions would not be counted toward the
total ratings in the results.
made some minor modifications to groups/import

In the Instructions section, the statement that tells instructors
to only use usernames or student numbers but not both has been
modified to say to use a student identifier which can be student
numbers or usernames.

For the formatting section, the Username column has been changed
to Student Identifier to avoid any unnecessary confusion.

I have added a help-text for the student identifier form field to
remind the user to select the student identifier used in the CSV
file.

The default student identifier has been changed to username from
student number.

For the error message when an unknown user is placed in the CSV
file, I have added an additional question which asks whether the
user has chosen the correct student identifier.
filtered out unrequired likert ques from total rating

The filter has been implemented for evaluation results for
instructors.
fixed mixeval copy

Due to the different structure being used for mixeval, it is slightly
more complicated to manimpulate the data to render mixeval/add.
To make it simpler, I have made a copy of the selected template
and redirect the user to mixeval/edit to make any changes.
fixed evaluation results total for students and instructors

I have changed the total_marks virutal fields to be only the sum
of all marks that belong to required likert questions.

I have also fixed the rating and penalty for student results for
mixeval.
fixed mixeval questions numbering

If the initial mixeval only has one question, and is being editted
the numbering of the questions restart at number 1. This bug has
been fixed. The reason was javascript translated index 0 to null.

On the side, I have also fixed mixeval index's total mark column.
When there are no questions in the template, the total mark column
is left blank, so I have made it 0 if no questions are found.
fixed mixeval edit, copy, and delete

When a mixeval is editted without any questions that have descriptors,
an invalid index error will occur. I've fixed that by only processing
descriptors if any exists. Another problem still exists when additional
questions are added when an a validation error occurs. When that happens
any new questions will disappear. We're still investigating this
problem.

I've fixed delete where it had an error of using a function that
was in evaluation_base. I've modified it to use it's own function.

I've modified copy again to make it similar to modifying simple
evaluations and rubric evaluations, where all of the parent
template's detail is copied onto an "add form".
fixed tests for events controller

The error happened because a new field (email_schedule) was
included in the events add & edit form.