Place `favicon.ico` in the public dir by default

[Filip Sodić]

Implement what Miho suggested here: #1622 (comment).

Instead of magically generating the default favicon.ico and telling users they can override it by placing their own in the public directory, we should just place the default one in the public directory. This change would:

• Allow us to remove the annoying .gitkeep file.
• Better communicate to our users how to change the favicon.ico (they trip over this every now and then, e.g., here and here).

What needs to be done:

• Change the favicon.ico generation (obviously :)).
• Update the docs.
• Decide how to handle the breaking change.
• Update the Changelog.

[Mihovil Ilakovac]

Some users wanted to use favicon.svg to set up their favicon, but our default favicon gets copied alongside theirs and overrules their svg favicon!

Having our favicon.ico in the public dir would prevent this from happening, although I'm not sure if users would need to setup their own meta tags in the head e.g. <link rel="shortcut icon" href="/favicon.ico" />

[Filip Sodić]

Both SVG and .ico are well-supported favicon formats. We should change Wasp to look for a file named favicon.ico|svg and template the head tag to use whatever it finds.