Desktop: add {{bowm}} and {{bows}} - Beginning Of Week (Monday/Sunday) (#4023)

* add {{bowm}} and {{bows}} - Beginning Of Week (Monday/Sunday)

In certain situation it is useful to reference the beginning of the week.
Unfortunately the current system does not allow to that, not even with a custom datetime format.

I've also added the 2 new template variables to the documentation.

* README: better wording