Add checkbox for "pay using staked SKILL only"

[Kevin Unger]

When user chooses to do action "mint weapon" or "reforge weapon"...

• When user selects "pay using staked SKILL only"...

Then the action is performed
And payment is deducted from user's staked SKILL (and *only* the staked SKILL), at a 20% discount

---

• When user does not select "pay using staked SKILL only"...

Then the action is performed
And payment is deducted the standard way, but with staked SKILL as another fallback (such that the order is `IGO -> Unclaimed -> Wallet -> Staked` - `Staked` is added at the end)

[Raymond Hammarling]

Closed because PR is merged.