Optimise Drawable/Mipmap resources for presence and size

[noln]

Seeing a number of missing resource issues, and and OOM, caused by missing or unoptimised resources, as well as some Tusky resources still in the codebase that needn't be.

1. Remove Tusky resources/replace with Roma-fied versions.
2. Replace PNG files with VectorDrawables where possible (which should be everywhere as minSdkVersion is 21).
3. (dependent on above) If any PNGs can't be replaced, optimise for size; e.g. running xxhdpi/avatar_default.png through TinyPng yields a 72% size reduction. Significant APK shrinkage potential if applied to all.

---

• Roma Version: 7.3

• Android Version: 5.0+ (21+)

• Android Device: * supported

• Mastodon instance (if applicable): N/A

• I searched or browsed the repo’s other issues to ensure this is not a duplicate.