[IMP] hr_timesheet: Improve the usability of timesheet portal

1. Generic timesheets improvements

    - Portal /my/timesheets
    - Add a group bys task, date, employee
    - Add search in project, name, employee, task
        * The task column should not be visible if none of the timesheets have the Task field set
    - If a group by task, date, employee or project is applied, the corresponding column should not be visible

2. Timesheets from sub-tasks on parent tasks

    - Portal /my/tasks
    - Display the 'Total Hours' spent on a task at the end of the list of timesheets
    - Below the list of timesheets for the task, display another table detailing the respective timesheets (and total hours) of each sub-task linked to this task (= 1 table/list per sub-task)
        * Title should be: 'Timesheets for sub-task: xyz'
        * only visible if there are sub-tasks linked to this task
        * only visible if total hours > 0

3. Timesheets on Invoices

    - Portal /my/invoices
    - Add Project + task fields
        * Task field only visible if set on one of the timesheet entries
        * Project field only visible if there are at least 2 different projects in these timesheet entries

4. Correct visibility for the portal users
    - Portal users is allowed to see timesheets if the users are added in allowed portal users list in the project
    - When a portal user is added as the message follower of the project, the user is added to allowed portal users as well
    - When a portal user is removed from allowed portal users, the user is also removed from message followers

closes odoo/odoo#47248

Taskid: 2201908
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>