Use `DEFAULT_CHAIN` everywhere it makes sense

[dnwiebe]

We recently changed the default chain from polygon-mainnet to base-mainnet. In order to do so, we changed the definition of DEFAULT_CHAIN, but there were a lot of specific references to polygon-mainnet (that is, not DEFAULT_CHAIN) that had to be individually changed as well. Many of them were in strings--for example, file paths.

Since the default chain has changed more than once in the past and will probably change again in the future, all the direct references to it should be changed so that they're references to DEFAULT_CHAIN instead. If that requires additional functionality to be added to DEFAULT_CHAIN so that it can render itself as a piece of a file path, then that additional functionality should be supplied.

[kauri-hero]

Perhaps should have been part of #685

Will make changing default changes in future much quicker