chore(bpmn): reorganize main page

Move bpmn main page from /bpmn.html to /bpmn/.
feat(camunda-modeler): add standalone modeler page
feat(build): add ability to include local css/js
feat(camunda-modeler): add download page

Related to CAM-1783
chore(download): add download link to alpha2
feat(analytics): add analytics via data-track tag
feat(camunda-modeler): improve SEO and accessibility

- moved modeler page to /bpmn/tool
- add modeler screenshots as separate images
- add google analytics linking
feat(camunda-modeler): add system requirements section
feat(camunda-modeler): blur out what is not important