Rebrand README and app strings to Iceraven Browser

[interfect]

This changes the user-facing strings and the README to say we are Iceraven and not Iceweasel.

This is part of #111.

[interfect]

Merging this in gave us a test failure:

   TEST: GIVEN sharedprefs contains installed engines WHEN installedSearchEngineIdentifiers THEN defaultEngines + customEngines ids are returned
    [Robolectric] org.mozilla.fenix.components.searchengine.FenixSearchEngineProviderTest.GIVEN sharedprefs contains installed engines WHEN installedSearchEngineIdentifiers THEN defaultEngines + customEngines ids are returned: sdk=28; resources=LEGACY
    [Robolectric] NOTICE: legacy resources mode is deprecated; see http://robolectric.org/migrating/#migrating-to-40
    I/AssetManager: no overlays.list file found
    I/MonitoringInstr: Instrumentation started!
    I [glean/GleanCrashReporterService] No crashes to record, or file not found.
  FAILURE

...

 Caused by: java.lang.NullPointerException
	at org.mozilla.fenix.components.searchengine.FenixSearchEngineProvider$refreshAsync$1.invokeSuspend(FenixSearchEngineProvider.kt:186)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
	... 5 more

[interfect]

I don't see how this would have caused that. I'm re-running the job to see if it's an intermittent failure or what.

See https://github.com/fork-maintainers/iceraven-browser/runs/1126607386