AdminPanel: When adding skills, only show options that are allowed for the class

[nitoygo]

Is your feature request related to a problem? Please describe.
When editing a character's skills, the drop down shows skills of all character class.
It's hard to know which skill is allowed for that class.
User also needs to know the skill names when using the search text box.
(There is also that 'Level' field which I'm not sure what for)

Describe the solution you'd like
When adding skills, only show options that are allowed for the class.
(This is probably a subset of things to do in #561)

Describe alternatives you've considered
Alternatively, list all skills with checkbox markers whether it is learned or not

Additional context