Split out app.js logic into separate modules

The app.js file has grown large as the single place where all data sources are declared. As of commit [86fdd5e](https://github.com/Blockstream/esplora/commit/86fdd5effd8dd5ad9beeaa3db4f466f4b3e1794c) this file sits at 600 lines and it will only continue to grow as we pull from more data sources. This issue is to separate the logic of this file out into different files as per [this comment](https://github.com/Blockstream/esplora/pull/633#discussion_r3545678321) in https://github.com/Blockstream/esplora/pull/633.