Visual regression testing

Add the infrastructure to keep track of visual changes to the rendered pages by taking screenshots of the pages and comparing them to reference baseline screenshots committed in the repository.

Running the testsuite with NEEDLE_SAVE_BASELINE=1 set in the environment regenerates the baseline screenshots, to be used when adding new tests or when introducing visual changes.

• 6f0f60db tests: Add visual regression testing
• 1039be7d tests: Check visual regressions with contents in the homepage
• 199985b6 tests: Cache initialized db across visual regression testcases
• d73981d7 tests: Capture a taller area for visual regressions
• 5aa6a447 tests: Ensure the testcase repo is available on the CI
• 7d6c7697 tests: Check visual regressions on the LXC report page