Update main yew examples (remove static dir)

[philip-peterson]

Description

Several updates to the main yew examples:

• Remove centralized static/ directory, in favor of each example directory's static dir
• Automatically start a webserver after building, and open in browser
• Update the index.htmls to load the generated JS properly

Fixes #1331

Checklist:

• I have run ./ci/run_stable_checks.sh
• I have reviewed my own code
• [NA] I have added tests

[siku2]

@philip-peterson Would you mind rebasing this?
I believe these changes are still worth adding even with the advent of #1419.

[siku2]

This will also break the ci/build_examples.sh script. That script isn't being used and if we want to add checks for the examples in the future we'll have to rewrite it anyway. I think we should remove it entirely.

I did some detective work and the script was added in #1165 where it was originally part of the CI but the same PR also removed it from the Travis configuration and it's been sitting there unused ever since.

[philip-peterson]

@siku2 Done!

[siku2]

This is a huge step-up from what we currently have. Thanks!

The reason there are so many comments for the Python file is because I want to keep the code style consistent.
I realise that it's kind of silly to be pedantic about it though so I only left comments with suggestions so they can easily be applied.
The file used both double and single quotes for strings. I replaced all instances with double quotes, not only because of my personal bias but also because it's more in-line with a Rust codebase.

[siku2]

I noticed that after stopping the server the script still prints:

[ Finished! Run EXAMPLE by serving the generated files in examples/static/ ]

as part of the EXIT trap.

Since I didn't want to hold this back another day I just made the edit myself. I hope you don't mind.

[siku2]

@philip-peterson I just noticed that the web server always adds ; charset=utf-8 to the content type header regardless of the mime type. The charset parameter should only be added to MIME types whose main type is text^[1].

This isn't a big issue, the examples still run perfectly fine. I only noticed because it logs a warning:

WebAssembly.instantiateStreaming failed because your server does not serve wasm with application/wasm MIME type. Falling back to WebAssembly.instantiate which is slower. Original error:
TypeError: Response has unsupported MIME type wasm.js:242:29

This is entirely on me for not noticing sooner, sorry about that.