[WIP] Bootstrap end to end tests with cypress

Summary:
Setup an environment to perform end to end tests using the [[ https://www.cypress.io/ | cypress ]] Test Runner.

Before running the tests, Software Heritage web application is executed in the
background using in-memory storages for fetching data contained in a sample archive.

To easily run the tests, two new make targets have been added:
- `test-frontend`: Run tests in headless mode (this should be used in a CI environment)
- `test-frontend-ui`: Run tests using the cypress GUI

As an example, a couple of tests regarding the highlighting of lines of code have been added.

Related T1768

Reviewers: #reviewers

Subscribers: shisus, kalpitk

Differential Revision: https://forge.softwareheritage.org/D1565