highlight not filled mandatory fields, check for sysdate

[arbadacarbaYK]

The time is mandatory. If not filled, the save-button for the competition does not show it with no warning.
Nicer UX would be to prefill the date&time-field to next day, same hour (from sysdate of manager).

Also time should in general not be possible in the past, so max 10min from sysdate at least (or smth like that).

[oren-z0]

Changed so far:

• Added a little "*" next to the required fields
• Made the description optional (can be left empty, like the banner url)
• Set the default "Max bet in sats" to 100,000,000 (I wonder how many lightning nodes can receive a whole Bitcoin as a payment).
• Set the default "Amount of tickets" to 100 - Admins can always increase that if they run out of tickets. Also changed label to "Number of tickets".

I've left the "Ticket closing date" as is. I want to completely refactor it to support local dates, not only UTC (i.e. use the browser's timezone, and specify it in a selection box or something).