Support custom CSL styles

[dstillman]

https://forums.zotero.org/discussion/110065/syncing-or-adding-custom-styles-in-ios

The style manager in settings should let you add a custom .csl file. Custom styles should have an indicator of some sort, and it shouldn't let you overwrite an official style (installed or not) with the same id.

Theoretically these should update automatically based on the link in the style, but I'm not sure the desktop app even does that now, so we can deal with that later.