Update Frost for Facebook to v2.3.1

https://github.com/AllanWang/Frost-for-Facebook/releases/tag/v2.3.1

The big change from 2.2.4 is that I no longer commit generated web files (css, js). As a result, those files went missing for 2.3.0. This version adds a gradle preBuild task that will generate the files for F-Droid. It should not require any extra dependencies as everything is done through gradle. Note that this is not the same as F-Droid's preBuild metadata.

To note, building the app will now cause new files to generate at app/src/web/assets and app/src/web/node_modules. Given that this is all during F-Droid's 'build' phase, I hope that no extra metadata is needed.

If possible, please check that the resulting apk has a css and js folder within the assets directory. (Or take a look at the log and check that something like this exists