[IMP] web: improve eval context dates

This commit handles 2 things:

1) the merging of the `py_utils.context()` into the evaluation context
returned by `BasicModel._getEvalContext` to access the same time-related
keys,

2) the addition of 2 new keys into the `py_utils.context` (thus
transmitted to the basic model): `today` (an alias for the already
present `current_date`) and `now` which represents the current date and
time value.

Task 2269697