Big codebase renaming

This codebase is doing a few things that aren't in line with the broader open source community, and I want to make it easier for people who contribute to many projects to hack on Soapbox with muscle memory. Here's what I'm thinking:

• Change the default branch from develop to main. This is what all projects use nowadays. Nobody uses develop, and we aren't doing anything special. So it just requires extra git configuration and goes against muscle memory.
• Rename app to src. Most tools like Webpack, Vite, etc assume your source directory is called src by default, and require extra configuration to call it app. Since src is well-known, it will improve the ability to contribute to Soapbox.
• Rename static to dist. The name static as an output directory never made sense, and once again dist is in line with the existing tooling as the counterpart to src.
• Make the zip artifact contain the root of the frontend files, rather than being inside a subfolder. This corresponds with the change above, and offers more flexibility about the directory you unzip into. It also makes it match other Pleroma frontends. We would change the CI job from build-production to simply build, so scripts using the old URL would still work, just wouldn't get updated.

This probably won't break a lot. A few people have custom scripts where this might matter, but I know them personally and can reach out about updating scripts. I think it will be worth it to make this change even though it will be just a tad bit painful. And it's definitely better to do it all at once and get it over with.