Use BAF logo as default NFT image

Currently the default image for an NFT minted to the contract is the NEAR logo. This should be an SVG BAF logo

Tasks
- [ ] Get/create an SVG version of the BAF logo and use a tool to optimize it (make it small)
- [ ] Set [DATA_IMAGE_NEAR_SVG_LOGO](https://github.com/bafnetwork/baf-badges/blob/5dcb954c59c9f438578bdf28611b34dbf8e262e3/contract/src/lib.rs#L40) to the BAF logo's raw SVG text and rename the variable so it's clear that it's the BAF logo.

Start in contract/src/lib.rs at the DATA_IMAGE_NEAR_SVG_LOGO constant linked in the issue, and inspect how it supplies the default NFT image. Create or obtain an optimized SVG BAF logo, replace the embedded SVG text, and rename the variable to reflect the BAF logo; done means newly minted NFTs use that logo by default.

Written by the indexing model from the issue text.