Added no same start/end time in shifts (with spec)

[sebjimenez]

Purpose or Intent

• The goal of this PR is to fix the following issue:
• Issue Add shift; start/end times cannot have the same value #18 (Start/End time cannot have the same value

For more information feel free to access the link to the mentioned issue.

[sebjimenez]

• This is how it would look at the moment. @agarciavera Please download the branch and provide feedback.
• *** An issue will be opened regarding several error messages displayed to the user at once.

[agarciavera]

Checked correctly in your branch. Rebase and I will checking again.