[chore] Add svg version of sloth logo as logo.svg

[tsmethurst]

Description

This pull request adds the svg version of the GtS sloth logo to the web assets folder.

relates to #969

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.