Reduce bundle sizes

[Treora]

Not so urgent optimisations to perhaps try some time:

• PouchDB could be bundled separately, and shared between background.js and overview.js. Same for React, if used in multiple pages.
• pouchdb-browser could be replaced with pouchdb-core & pouchdb-adapter-idb (& pouchdb-mapreduce`?)
• Replace simple lodash functions. Even importing only mapValues draws in a whole bunch of its friends. Useful to depend on while prototyping, but they could perhaps be removed again.