fix: Close #82 Enforce users manipulate only their time entries

[EliuX]

In this PR we make possible change the model of the time entries, so that it is checked that the current user can only manipulate time entry items where he is the owner, otherwise he will get an 403 error:

This is possible because of the new parameter peeker and conditions:

• peeker: It is a peeker that is meant to watch the item that is manipulated and if a biz-logic criteria requirement is not fulfilled it can short-circuit the user request flow by raising an CustomError.
• conditions: It is a python dictionary that contains the attribute and the corresponding expected value we expect the entries have in the database. This is done by creating an equality condition in the where clause of the SQL query executed in the function find_all.

[Angeluz-07]

Good Job.