Allow creating new characters
Adding flash notices
Adds xp to characters

This adds the notion of xp to the characters. It also introduces the
`Character#can_level_up?` method, which evaluates the
`GameSystem#xp_algorithm` as a math expression, and compares it to the
character’s xp value, to determine if the character can level up.

When assigning a game to a character without any prior xp, the game’s
`initial_xp` will be transferred to the character.

Additionally, this also increases the `GameSystem#description` from a
string to a text type, allowing a greater amount of text to be stored.
Adds Levelup controller

This introduces the `Levelup` wizard controller, and the
`Levelup::<step>Step` steps to validate this wizard.

When level-ing up your character, this is the controller responsible
for keeping track of the flow.