[WiP] feat: backend for thumnail computation + caching + endpoints

[mistercrunch]

This PR is the backend subset of WiP PR #6601. It includes:

• refactor of the Selenium abstractions used for email schedules,
  generalized to also work with thumbs
• new dependency on PIL, the common way of doing image processing in
  python, we use it to resize selenium screenshots into thumbs
• CLI utilities to compute-thumbnails
• the addition of an extra caching backend for thumbnail, in most cases
  we assume it should point to the same backend as the one used for
  chart JSON caching
• 2 new endpoints to retrieve dashboard and chart thumbs

CATEGORY

Choose one### SUMMARY

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

TODO / extra thoughts

• The security on viewing thumbnails should be the same as viewing the chart/dashboard itself
• We should perf/load test viewing a pageful of thumbnail. Assuming ~50 thumbs are displayed on a card list view, how does the metadata database react? It can represent a fair amount of database lookups depending on the way that perms are setup (if the user is Gamma, and there are lots of dashboards on the system). We should make sure that the worst case is ok.
• Caching for thumbs can be lax (doesn't have to be done daily for instance), but it's different for the image service (say for Slack or email scheduled delivery). Caching for thumb is currently forced on save events (async, and that's the way we'd tell users to get a new fresh thumb), or we could add a feature to force on the endpoint. There's also the CLI. For caching for other purposes, I think that the caching policy for charts should apply for the image the same way as it does for the JSON caching (same logic/policy).

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[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions. For admin, please label this issue .pinned to prevent stale bot from closing the issue.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions. For admin, please label this issue .pinned to prevent stale bot from closing the issue.