Many small improvements to the events form

[aykevl]

Commit:

• Only allow setting the 'name' property on creation, not while editing (to not break URLs).
• Reverse 'name' and 'humanName', so 'humanName' comes first (more logical for humans), and change their labels to be easier to understand.
• Default 'cost' value of 0.
• HTML5 required attribute for most inputs.
• Some HTML5 placeholder values

Maken en bewerken van een event is getest. Ik hoop dat deze commit het aanmaken van activiteiten wat makkelijker maakt.

Fixes #332

[bwesterb]

Dank je! Staat online.