Add Iceraven Browser

This adds the Iceraven Browser, a more customizable and less telemetry-heavy fork of Mozilla Firefox's Fenix version.

@relan I'd appreciate your help with this. I have it to the point where it tries to build geckoview, but I run into a complaint that the Gradle version is too old:

1:17.84 Build file '/z/home/anovak/workspace/fdroiddata/build/srclib/MozFennec/mobile/android/geckoview/build.gradle' line: 5
 1:17.84 * What went wrong:
 1:17.84 A problem occurred evaluating project ':geckoview'.
 1:17.84 > Failed to apply plugin [id 'com.android.library']
 1:17.84    > Minimum supported Gradle version is 5.1.1. Current version is 4.4.1. If using the gradle wrapper, try editing the distributionUrl in /z/home/anovak/.gradle/daemon/4.4.1/gradle/wrapper/gradle-wrapper.properties to gradle-5.1.1-all.zip
 1:17.84 * Try:
 1:17.84 Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
 1:17.84 * Get more help at https://help.gradle.org
 1:17.84 BUILD FAILED in 43s

I remember seeing something in your fennecbuild repo about patching required Gradle versions, but you redid the whole repo and it seems to be gone now. Can you explain what you were doing with regards to Gradle versions and why you stopped doing it?