Refactor of API timeline actions (#3263)

- Increase coverage to exercise all parts of each action
- Move into namespace to share common code
- Misc refactor of each action for smaller methods, simpler code

spec/routing/api_timelines_spec.rb

@@ -0,0 +1,18 @@
+require 'rails_helper'
+
+describe 'API timeline routes' do
+  it 'routes to home timeline' do
+    expect(get('/api/v1/timelines/home')).
+      to route_to('api/v1/timelines/home#show')
+  end
+
+  it 'routes to public timeline' do
+    expect(get('/api/v1/timelines/public')).
+      to route_to('api/v1/timelines/public#show')
+  end
+
+  it 'routes to tag timeline' do
+    expect(get('/api/v1/timelines/tag/test')).
+      to route_to('api/v1/timelines/tag#show', id: 'test')
+  end
+end