Show available margin in Add/Remove margin modal

[kappacappa]

In the Add or Remove Margin modal, show the user's wallet balance when they're adding margin, and show how much margin is available to remove if they're removing margin.

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Bounty

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[kappacappa]

@MrKevin0308 this would require to open a position, then to click on the Edit (pencil) icon. You can open a position by submitting a market order on any market.

[kappacappa]

@MrKevin0308 we can't do that at this time. I'd suggest picking another issue for now.

[hjjinx]

Requesting to work on this

[kappacappa]

@hjjinx assigned.

[KSSaiTeja]

@kappacappa i think that the issue is still open.. can you please assign it to me..

[AnkushBL6]

Hi, I'd like to request assignment for this bounty if it's still available. I opened a PR here: #111

[autochamchikim-pixel]

Requesting assignment/consideration for this bounty. I opened PR #141 to resolve it: #141