[Cashtab] Rename getMaxMintAmount to getMaxDecimalizedSlpQty

Summary:
This function is not really giving us the maxMintAmount. It's giving us the max decimalized qty representable for a given SLPv1 mint, send, or burn tx.

In Cashtab right now, it's only used for getting a max mint amount. But we need to fix this.

This amount should also be added to max mint/send/burn validation. Need to do this to properly validate agora partial listings.

In this diff, we only rename.

Test Plan: npm test, grep -r getMaxMintAmount src/ and no output

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16949