Feat/#849 manage employees

[IronMan756]

Added date picker to "edit-employee-employment.component", added field "startedWorkOn" in DB, reorganize inputs in "edit-employee-employment.component", tab Hiring was relocated
video:

https://www.loom.com/share/db9baf7af9944e8a83d44e1be0f8a669

[evereq]

@IronMan756 looks good, I will merge it now, but can you please:
a) fix DeepScan issues
b) watching your video it looks like you made that Work Start Date field kind of required? Because the "Save" button was disabled before you enter some value in start date input field. Maybe it was not clear in the task description, but we need that value to be optional and we just show that notification to user (warning) that some features will not be available, but it should be possible to SAVE form without that date (e.g. edit some other fields and save the form with an empty date for the start of work).
c) Please put It's required to enter the date when the employee started work to generate employee payroll, enable the employee to participate in split expenses, see employee statistics, etc as translation (into JSON file), similar to all other text

You can push all changes to the same branch and create again PR so we can merge it again.

Thank you, good work!