Some suggestions for access token expiration selector

[mayorova]

While reviewing #3943 I saw some things that I thought could be improved, but I was not sure exactly what would be a better way, so I played around a bit.

Probably not everything should be applied, but can be considered as food for thought.

Mainly the changes are:

• Using helper text (under the select) instead of the custom span element to the right (which dos not look very Patternfly-y). The drawback is that when the select is expanded, the value is not seen, but I think it doesn't matter.
• Changing the formatting of the "current value". Not sure it's a nice one (probably too long), but I think it's clear, and it also includes timestamp (but not sure if needed)
• When the date is selected via picker, the selected value is also shown now.
• The default value when using "Custom..." is set to tomorro
• The past dates are not pickable in the date picker.

In addition to the last item, it would probably be nice to add an error message when the expiration is set to some time in the past. With this validator it is probably not that relevant, but if the past dates were esasily pickable, an error message saying that past dates can't be selected would be nice (but not required).