Replace dangerfile with eslint plugins

We currently use dangerfile only for jasmine fdescribe/fit tests and
Angular onPush checks.

Both can be replaced by eslint plugins.

Dangerfile

@@ -1,24 +0,0 @@
-# Fail if jasmine specs contain fdescribe or fit
-fail("jasmine fdescribe/fit left in tests") if `grep --include '*.spec.ts' -rP 'fdescribe\\(|fit\\(' frontend/src/`.length > 1
-
-# Search for modified components not being made OnPush
-git.modified_files
-    .select { |path| path.include?('frontend') && path.end_with?('.ts') }
-    .each do |path|
-  next unless File.readable?(path)
-
-  lines = File.readlines (path)
-
-  # Ignore non component files
-  component_line = lines.grep(/@Component/)[0]
-  next unless component_line
-
-  # Check for missing onPush
-  unless lines.grep(/changeDetection:\s+ChangeDetectionStrategy.OnPush/).length > 0
-    warn(
-      "Please use `ChangeDetectionStrategy.OnPush` for this component",
-      file: path,
-      line: lines.index(component_line) || 0
-    )
-  end
-end