Release 2.19 docs update

[jtojnar]

Release news: https://github.com/fossar/selfoss/blob/release-2.19/NEWS.md#219--unreleased

• [x] Drop Heroku/Deis mention from installation
• [x] Point Docker image to an updated one (pending owner approval)
• [ ] Ensure the docker image works https://gitlab.com/radek-sprta/docker-selfoss/-/issues/4
• [x] Update Android app link (pending upload to F-Droid)
• [x] Fix search button style since it looks bad on screenshots. https://github.com/fossar/selfoss/commit/fa0aeda33abb9a636cec21809281c1e38131afac
• [x] Do not create a service worker before enabling offline mode (https://github.com/fossar/selfoss/pull/1361)
• [x] Go over the installation instructions once more (https://github.com/fossar/selfoss/issues/1293)
• [x] Update screenshots on the website.
• [x] One last dependencies bump.
• [x] Go over the news.
• [ ] Bump the version.

Fixes: #1363, #1293

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